.5^-
PROBLEMS OF CYTOLOGY AND EVOLUTION
IN THE PTERIDOPHYTA
The motile spermatozoid of a fern {Dryopteris I'illarsii from Cyprus) killed with osmic vapour and
photographed with ultra-violet light at a magnification of three thousand diameters. The organs
of locomotion are numerous cilia, each of which is a tuft of still finer threads. The body of the
spermatozoid appears black because it is almost entirely composed of the nucleus which absorbs
ultra-violet light strongly. In life the body is tightly coiled.
A
- »
PROBLEMS OF
'7
CYTOLOGY AND EVOLUTION
IN THE PTERIDOPHYTA
BY
I. MANTON
Professor of Botany in the
University of Leeds
CAMBRIDGE
AT THE UNIVERSITY PRESS
1950
PUBLISHED BY
THE SYNDICS OF THE CAMBRIDGE UNIVERSITY PRESS
London Office: Bentley House, n.w. i
American Branch: New York
Agents for Canada, India, and Pakistan: Macmillan
Printed in Great Britain at the University Press, Cambridge
{Brooke Crutchley, University Printer)
CONTENTS
Preface page vii
Chapter i. Introduction to the Method i
2. Introduction to the Problem
3. The Polyploid Series in Osmunda
4. The Male Fern Dryopteris Filix-mas
5. The Genus Dryopteris in Britain
6. The other British Ferns — Polystichuni, Athyrium, Asplenium, Ceterach
7. The other British Ferns {continued)
8. Poly podium vulgar e
9. Three Special Cases of Fern Hybrids : Scolopendrium hybridum, IVoodsia
10
1 1
12
^3
14
16
17
and Polystichum illyricum
Apogamous Ferns. The General Phenomenon
Apogamous Ferns {continued). Evolution of the Separate Species
Induced Apogamy
The Genus Equisetum
The Psilotales
The Lycopods (Clubmosses)
The Ancient Ferns
Conclusions
Appendix i. Notes on the Cytological Technique
2. Notes on the Photographic Technique
3. General Summary of the Principal New Facts Recorded
4. Complete List of Chromosome Numbers
Bibliography
Index
13
26
44
63
88
1 10
127
142
158
171
196
209
233
244
262
281
293
298
300
302
306
311
65578
PREFACE
The publication of this book is one of the more harmless consequences of the Second
World War. The scientific work which it contains was begun in 1932, at first with no
clearly defined programme but rather as a hobby in relation to a field botanist's
interest in some attractive British plants, although in part also as a salutary exercise
and test of skill. For the Pteridophyta are difficult cytological material. They keep a
cytologist on his mettle the whole time to an extent that the student of the more familiar
Flowering Plants only rarely experiences, and only by patience, skill and the application
of the most modern methods can success be achieved. It is, indeed, no accident that,
in spite of three-quarters of a century of 'modern' cytological research by many
workers to whom at first the Pteridophyta were familiar and much used material, there
is at present practically no single species except Osmunda regalis for which the available
published accounts can be accepted as accurate. This much will at once be clear to
anyone who takes the trouble to compare the photographs reproduced in this book with
the quotations from the literature in even a recent compilation such as that of Love
and Love (1948). To remedy this state of affairs individually for each species to be
handled has therefore been a problem in itself, and only after the inquiry had been
in progress for some years did the several parts of it begin to take on the coherent
form of a wider investigation on the lines indicated in the first two chapters. Some
of the episodes, notably the study of autopolyploidy in Osmunda (Chapter 3), were at
an advanced state of completion fairly early, since in this particular case the inquiry
had been used as a preliminary study, on material of known origin and with fairly
large chromosomes, to serve as a background to the elucidation of cognate problems
in other groups. The published accounts which subsequently appeared, of the flowering
plants Biscutella and Nasturtium, undoubtedly benefited greatly by this procedure,
though the work on Osmunda itself remained unpublished. Other subjects, notably
the taxonomic analysis of the Male Fern (Chapter 4), were undertaken as it were
involuntarily, having arisen as an unexpected complication in an inquiry intended
to elucidate the cytology of apogamy. This in turn led on to the wider consideration of
the British species oi Dryopteris (Chapter 5) and so on. Being incidental to other work,
very little had been rounded off for publication before the war broke out and, indeed,
the only significant item had been the appearance of a preliminary note on the Male
Fern published in Nature in August 1939, in reply to a longer paper by Dopp on the
same subject, which had been received in July of that year. Immediate publication of a
fuller statement on the Male Fern work would have followed the appearance of the
preliminary note, and this would have been succeeded by serial publication of the
other subjects but for the outbreak of hostilities in September 1939. The gravity of the
international situation then made it appear somewhat frivolous to continue such a
programme at such a time, and as a matter of deliberate policy throughout the war
years publication was only attempted for work of exceptional importance. As regards
vii
PREFACE
the evolutionary studies in. the Pteridophyta, this amounted only to a short 'Note on
the Cytology of Psilotum with special reference to Vascular Prothalli from Rangitoto
Island', which appeared in the Annals of Botany in 1942, since in that case the possible
destruction by enemy action of the unique material supplied by the late Dr Holloway
of New Zealand, or of the manuscript containing the cytological description of it,
would have constituted a serious loss to science. As the war advanced, continuance of
the observational work became more and more difficult, and a large programme of
genetical inquiry which had been planned to amplify the results for the Male Fern and
other species had largely to be abandoned. Many valuable plants were lost either in
air raids or from neglect. Nevertheless, at the close of hostihties a sufficient thread of
continuity had been maintained to produce the situation that whereas in 1939 some
half-dozen draft manuscripts of projected papers had been held back for further work,
in 1945 these and several other topics had been developed as far as could reasonably
be expected with the material available in England. The congestion which would
result from the simultaneous presentation of such a large number of papers to the
depleted scientific journals of the immediately post-war years was, however, painfully
apparent, and for this reason book form was first envisaged.
Anyone who has shouldered the task of reducing to manageable proportions the
sciendfic notes of fifteen years' observation on more than this number of different
topics will perhaps treat the present author with sympathetic forbearance. Without the
constant help and encouragement of many friends and colleagues in the University of
Manchester and more recently in Leeds, exhaustion might well have set in and the
project have been abandoned. Not only has it been a matter of clinching the observa-
tions on literally hundreds of the troublesome details which remain uncertain long
after the broad outlines of results have been safely established, but the mode of
presentation is somewhat unfamiliar. The potential reader of a scientific paper can be
judged with reasonable certainty and the style of writing chosen accordingly. The
reader of a book is more unpredictable, and whatever modifications of style are adopted
to adjust the subject-matter to book form it is almost inevitable that certain parts will
be too simple for some readers and too difficult for others.
To what extent the author will have succeeded in the attempt to write simply is for
others to judge, but in the hope of making the work understandable to as wide a circle
of readers as possible, most of the essential concepts and such technical terms as are
unavoidable are introduced by means of illustrative examples in the first three chapters,
and the only mental equipment which is presupposed in the reader is an elementary
acquaintance with simple Mendelism and some general awareness as to what is meant
by a chromosome. To a botanist who has this equipment as a matter of course a
glance at the illustrations will convey the factual content of Chapters 1-3, and the
book may be said to begin in earnest at Chapter 4. A zoologist interested in general
evolutionary studies, and similarly equipped but lacking an intimate acquaintance
with plants, would do well to begin at Chapter 2. A field naturalist, however, whose
primary interest is not the laboratory but the life of plants in the field, may need
to read parts of the first three chapters twice over, after which he should have no
more difficulty in understanding the rest of the book than in reading, for example,
viii
PREFACE
Dr Turrill's volume on British Plant Life inihe New Naturalist Series (No. lo, 1948),
a book which may indeed be recommended as an excellent introduction to the present
study.
With regard to the illustrations a word of explanation should perhaps be offered as
to their purpose and use. In a group hke the Pteridophyta, where the technical
difficulties are so great that it has been my unfortunate lot to have to correct errors in
the work of almost every previous investigator, the attainment of accuracy has been a
primary task without which no vahd general conclusions could have been drawn.
For this reason the use of photography has assumed a special importance. The data
themselves have in the first place been assembled as far as possible according to the
principle that what cannot be photographed cannot be used as evidence. The photo-
graphic evidence so assembled has then been utilized to provide illustrations to the
book on a scale which should enable the reader to repeat for himself upon the printed
page enough of the observations quoted to judge of their validity. Only so can finality
in the establishment of basic facts be hoped for, and that many errors have been
removed or prevented by this procedure is certain. That some errors may still remain
is, however, only too probable, although where uncertainty is known to exist it is
unlikely to exceed the limits indicated in the text.
All details regarding technical methods are given in the Appendix under the two
heads of cytology and photography. In cytology, more perhaps than in any other
science, progress depends on manipulative skill and that type of low cunning which is
needed to apply old methods to new uses. A considerable range of methods old and
new will be found illustrated throughout the book, from which it will perhaps be clear
that the shortcomings of previous workers, to which attention has so often to be directed,
have been due in the main to the shortcomings of their tools. Standard techniques such
as that of sections stained in haematoxylin or gentian violet are indispensable for certain
purposes, and when used with precision in easier material can yield all the information
required. This is not, however, the case in the Pteridophyta. Sections are indeed of
great value as supplementary evidence and for comparative and morphological
purposes, but except in very rare cases, the Osmundaceae and Selaginella being the only
ones known to me, sections alone are inherently unsuitable for accurate cytological
work. Only by the squash methods, either with Feulgen staining or with aceto-
carmine or other reagents, can accuracy be reached. In some groups with large
sporangia, such as the Osmundaceae, Eusporangiatae, Lycopods, Horsetails and so on,
smear or squash methods as devised for flowering plant stamens can be used without
modification. The Leptosporangiate Ferns, however, which form the subject-matter
for more than half the book, have sporangia far too small for this. With only 8 or 16
mother cells in each it is impossible to handle a sporangium singly, and the fact that
they grow in sori in which many developmental stages are always present together offers
what at first sight seems like an insuperable obstacle to the use of the simpler smear
methods. The success, such as it is, which is claimed in the present study is primarily due
to the overcoming of these preliminary difficulties as a result of which the power of new
techniques has become fully available, and it should be found that, undetected accidents
apart, in the main where accuracy is claimed, even in species like Cystopteris with n = 1 26,
IX
PREFACE
the facts are as fully authenticated and as firmly established as is the chromosome number
of the Broad Bean.
In an extended inquiry all aids to accuracy which can relieve the observer of personal
fatigue are of great importance. In this connexion the photographic methods (Appendix
2) used for the making of black and white diagrams are perhaps of some interest.
Practically none of the numerous black and white illustrations of fern morphology and
of chromosomes has been based on a drawing of the usual type, and in dispensing
entirely with the old-fashioned camera lucida both speed and accuracy have been
greatly improved.
With regard to the identification of material, this has not often been in any doubt,
although as a precaution close contact has been maintained with the herbaria at Kew
and the British Museum throughout the work. Since, however, it may perhaps be of
importance to later investigators to know more about the actual specimens used, a
herbarium specimen has been retained for almost every species, access to which may be
had on request either direct to the author or to the authorities at Kew.
With regard to nomenclature, a perennial difficulty, this has become more acute
during the preparation of the manuscript owing to the numerous changes in taxonomic
usage regarding familiar species which have occurred since the work began. The
Check List of British Vascular Plants, published under the auspices of the British Ecological
Society (Clapham, 1946) in preparation for the compilation of new British Floras, has
emended terminology partly in accordance with preliminary communication of the
cytological results (as in the Male Fern complex, Manton, 1939) but in part inde-
pendently of this. More extended works, such as the Genera Filicum of Copeland (1947)
or th& Revised Classification of the Leptosporangiate Ferns by Holttum (1947), have similarly
affected the views previously current on questions of phylogeny. Since one of the aims
of the cytological study is to provide independent evidence on taxonomic matters it has
been felt that to revise the work constantly to incorporate these various developments as
they appeared was neither necessary nor desirable. As a basis of discussion any system of
classification and naming will suffice provided only that it is familiar and unambiguous.
Both these conditions are fulfilled by the very familiar phyletic views of F. O. Bower
and by the system of nomenclature of a standard Flora such as Babington's Manual of
British Botany (nth edition, 1922). At the time the work began these were all that were
available, and they still seem suitable as a framework on which the cytological facts
can be disposed. Emendations which seem necessary in the light of the cytology itself
can then be clearly distinguished and their agreement or disagreement with newer
views based on other evidence assessed. For ease of reference, however, the more
important cases in which the integrity of species has been disturbed by the cytological
facts are listed in Appendix 3, and a complete list of new chromosome numbers is
added in Appendix 4.
That the work is incomplete is self-evident and a defect for which apology cannot be
made. The stopping place in an extended inquiry is in a sense arbitrary and could
be indefinitely postponed if it were made to wait upon completion of every item raised.
One can only repeat with Goethe that 'Die Kunst ist lang; und kurz ist unser
Leben', and since the ground which can be covered by the labours of one worker is
PREFACE
narrowly circumscribed of necessity, one should perhaps be thankful that in the midst
of the difficulties of this atomic age any progress has been made at all.
It would be invidious, in a work which owes such a heavy debt to others, to select
many names for special mention. An exception must, however, be made for Pro-
fessor W. H. Lang, without whom the work would not have been done at all. And of
the many contributors who are not my colleagues, I must record a special debt to
Dr R. L. Praeger of Dublin, Mr F. Ballard of Kew, Mr A. H. G. Alston of the British
Museum, and Dr B. T. Cromwell of Hull. For personal assistance I am specially indebted
to Professor Lang's former research assistant, Mr Ashby, for help over the many years
I spent in Manchester, and, more recently, to Mr B. Clarke for similar assistance in
Leeds. Finally, I am indebted to the Oxford University Press, for permission to
reproduce various figures from my own papers in the Annals of Botany and the Journal of
Experimental Botany, to the Royal Society for similar permission regarding papers in
their Proceedings and Transactions, to Professor R. Nordhagen of Oslo and Professor
P. Martens of Louvain for permission to use Figs. 49 and 142 respectively, and to various
friends and colleagues for the originals of photographs which have been reproduced
as figures, notably to Professor Lang for Figs. 8a, 12 and 143, to Dr H. F. Dovaston
for Fig. 98 and to Mr Ashby for many of the natural-sized and low-power photo-
graphs throughout the book.
L MANTON
Leeds
December 1949
CHAPTER 1
INTRODUCTION TO THE METHOD
A scientific historian writing at some future time about the growth of biology in the
nineteenth, twentieth and perhaps twenty-first centuries might be expected to begin his
account somewhat as follows :
'The nineteenth century was a period of great intellectual activity in biology,
carried on it is true by a limited number of people but containing among them some
of the most powerful minds of the age. The perfecting of the compound microscope
was the most significant technical development, by means of which, for the first time,
animal histology and plant anatomy, together with the descriptive facts of life history
of the principal organisms of both kingdoms, could be adequately explored. On the
theoretical level the theory of evolution, coming as it did concurrently with this, and
based as it was in the hands of Darwin on the accumulated content of several centuries
of morphological observation of living and extinct plants and animals, wound up as
it were thejDurely descriptive phase of these sciences and paved the way for the experi-
mental period which was to follow.
'The effect of the theory of evolution on human thought as a whole was revolutionary
and in many ways catastrophic, but this aspect of the history of ideas need perhaps not
be discussed by the merely scientific historian. Within biology itself the effect, though
actually profound, was superficially less marked. The great achievements of the
pre-Darwinian period, namely, the establishment of workable principles of nomen-
clature, description and classification, together with the recognition of the nature of
fossils and of their use for the compilation of the geological time scale, remained almost
unaffected in their factual content although profoundly influenced by the new con-
ception of their purposej( The quest for phylogenetic significance became a conscious
aim to be pursued almost to the exclusion of all else towards the close of the century, and
under it the descriptive aspects of morphological biology expanded rapidly) with the
renewed vigour and interest which the change of aim awakened. The methods by
which this aim were pursued remained, however, substantially the same as those
available to Darwin himself until, with the twentieth century in sight, the change from
observation to experiment set in.
'This change was closely associated with the gradual recognition, not by isolated
individual workers but by the scientific world as a whole, of the need for an objective
study of the facts of variation as the next step required by the Darwinian theory of
evolution. This need was first effectively voiced by Bateson in the celebrated intro-
duction to the Materials for the Study of Variation of 1894, and it led directly to the re-
discovery in 1900 of the work of Gregor Mendel.
'The effect of Mendel's papers at this their second -appearance was at first to obscure
the connexion between evolution and the study of variation owing to the necessary
preoccupation of geneticists for the next few decades with establishing the rules of
MFC T I
INTRODUCTION TO THE METHOD
their craft by an extension of the Mendehan experimental methods. The evolutionary-
context, however, came back into the picture after the unification of genetics with the
sister science of cytology, which occurred in the period between the first two world
wars. This was made possible in the first instance by the introduction of team work
into genetics, notably by the American school of workers on Drosophila founded by
\ Morgan in the 1910's, but the final proof that the chromosomes are the seat of Mendelian
\ inheritance was first conclusively given in 1931 simultaneously for an animal, Droso-
phila (Stern, 1931), and a plant, Z^a mays (Creighton and McChntock, 1931).
'The resulting establishment of cytogenetics as an exact science must be recognized
as one of the biggest intellectual achievements of the first half of the twentieth century.
The roots of cytology, as of genetics, can be discerned in the nineteenth century,
some well-known landmarks being 1875, the date of the publication of ^ellbildung
und ^elltheilung by the botanist Strasburger with its factual description of mitosis,*
and 1894, the numerical demonstration of chromosome reduction by the same author.
Nevertheless, we know that cytology no less than genetics was a twentieth-century
science from the fact that almost everything in Strasburger's 1894 paper was erroneous
except the basic numerical conclusion. It required the pioneer work of Gregoire in the
years preceding 19 10, Janssens, Belling and others in the 1920's, and many more
workers, both earlier and later, to stabilize technique and to elucidate the funda-
mental descriptive facts of meiosis * without which cytogenetics could not have been
established.
'By this means as the century advanced, a new tool of great and unexpected power
was made available for students of evolution. It became possible to investigate the
nature of species, or at least of some species, experimentally, to diagnose their mode
of origin and trace with precision some significant parts of their genealogy. The pre-
occupation of biologists with tracing phylogeny on the grand scale gave place to the
attempt to analyse some actual evolutionary mechanisms by experimental means.
The success which attended these efforts was, in the first place, of importance as a
direct proof, if proof were needed, that the idea of evolution represented not a theory
but a fact, and in the second place to shatter the Darwinian conception as to ways and
means.
'The destructive effects of the new knowledge on the compelling simplicity of
"Darwinism" came from the recognition that its apparent simplicity was over-
simplification, and that what was next req^uired was not one generalization to account
* An elementary acquaintance on the part of the reader with the basic facts of mitosis and meiosis
is here presupposed, although it is sufficient at this stage to know merely that mitosis is another name for
nuclear division of the ordinary kind, while rneiosis is a peculiar form of nuclear division, sometimes
referred to as reduction division, which occurs at one point only in the life cycle of every species which
reproduces by sexual means. In mitosis the chromosomes split longitudinally and their number remains
constant throughout the process. In meiosis there are always two nuclear divisions in rapid succession,
in the first of which the chromosomes pair and then separate to opposite poles so that their number
is halved in each of the resulting nuclei. A detailed knowledge of the mechanism of meiosis is not
required for the purpose of this book, but such parts of it as are essential, notably some details of
chromosome pairing, will be described in relation to illustrative examples in Chapters i and 2 and
especially Chapter 3.
i
INTRODUCTION TO THE METHOD
for the origin of species in the singular, but a painstaking analysis of numerous special
cases of the ori.gins of species in the plural, carried out objecli\cly and without undue
deductive reasoning until sufficient wealth of well-authenticated individual cases should
have been assembled to make a fresh generalization possible. This programme of
work took the rest of the twentieth century to carry out and was still occupying the
^attention of many minds in the middle of the twenty-first.
'By this time', our historian might continue, 'two further events had occurred. The
effect of the visual light microscope on the scope of work of the nineteenth century was
repeated at a different level vijijthe twentieth by the perfecting of the electron micro-
scope. This brought the field of molecular structure under direct observation, and
made possible for the first time a full description of organic materials. Simultaneously,
a synthesis was effected between cytogenetics and the previously independent science
of enzymology, so that the dynamics of living matter could at last be explored. The
consequences of these developments were naturally jiot fully appreciated until the
_twenty-first century was well under way, although the shadow of big things to come was
clearly discernible by the middle of the twentieth.'
Taking leave of our historian while yet there is time and before he intoxicates us
with a preview of distant scenes, we may ask in sober earnest what has in fact happened
to the theoretical background of knowledge in almost a century which has elapsed
between publication of The Origin of Species in 1859 and the present day? As I see
it, the change in our attitude to theories of evolution rests principally in a new precision
which can now be attached to the words 'variation' and 'variability'. In Darwin's
day these concepts were so ill-defined that plausible assumptions, uncontrolled by any
reference to experiment, could at any time be made without serious challenge except
on a priori grounds. At present we possess enough exact knowledge about both terms
to be able to discuss the basic concepts of this or that general theory of evolution, not on
logical grounds alone, but to some extent on a basis of fact. The underlying assump-
tions required by simple Lamarckism (evolution by use and disuse), or simple Darwin-
ism (adaptive evolution by gradual accumulation of minute heritable differences)
or the mutation theory first voiced by Darwin's earliest opponent, Richard Owen, and
later much elaborated in the hands of de Vries and others (evolution by sudden leaps),
can all now be seen in their original form to be over-simplifications. They may con-
tain a greater or less germ of truth, but they cannot be the whole truth, for we now
know enough to be certain that variation is not one process but many; different types
of variation have widely different causes and consequences, and all follow their own
laws of behaviour which must first be elucidated before they can safely be built into any
theoretical scheme. The task before us is therefore seen to be something quite different
from that with which the theorists of the last century were concerned. We have a
new field of knowledge to explore, and the exploration is only just beginning. Exactly
where it will end cannot at this date be wholly foreseen, and it may therefore be well
at the outset to disinterest ourselves from general theories in order to concentrate the
better on a limited number of rather fundamental questions. How many types of
evolutionary activity can we actually detect? How do these differ and what are their
characteristics? What proportion do the analysable cases bear to the unanalysable?
Q i-a
INTRODUCTION TO THE METHOD
And what, lastly, are the accumulated effects of each type of evolutionary change
likely to be if carried on and repeated over a span of millions of years? Only when some
reply has been obtained to this last question shall we be in a position to assess the real
power of our present tools and to judge whether or not a generahzed evolutionary
theory can in fact be constructed.
It may be said at once that this stage will not be reached in the course of this book,
nor, in the opinion of the writer, is it to be looked for for many years to come. In the
meanwhile we may cultivate our garden, but before doing so it may perhaps be of help
to the uninformed reader, if such there be, to explain a little more precisely what it is
that cytogenetics at its present stage of development can do.
Genetics alone can contribute much to an understanding of the differences which
separate natural units of less than specific rank. It is true that inquiry is much restricted
by the difficulty in most cases of getting behind the necessarily vague concept 'genie
mutation'. Sometimes we can determine the place on a chromosome where a 'muta-
tion' has occurred. In other cases we can measure some statistical facts about its
frequency of recurrence and can sometimes alter this frequency by deliberate inter-
ference (induced mutations). As a rule we do not know at all what has occurred unless a
piece of chromosome large enough to be seen has become lost or misplaced. We are
likewise generally ignorant of how the mutation acts to produce its visible effect. The
effects can, however, be studied, their distribution in the progeny of crosses analysed
and predicted, both under controlled conditions and to some extent in natural popula-
tions, and the accumulated knowledge so gained can in favourable cases give us the
basis of a numerical idea of the relative complexity of genetical differences which
separate one natural form from another. These natural forms are, however, very rarely,
if ever, species, and as a general rule, genetics, unaided by cytology, is unable to
extend its analysis beyond the level which Goldschmidt has fittingly labelled ' Micro-
evolution' to contrast it with ' Macroevolution', on which alone the attention of the
older evolutionists was bent.
Cytology is, however, in somewhat better case. With the knowledge that the chromo-
somes are the seat of genetically active materials and that changes in these are the
physical basis of evolution, the comparative study of chromosomes, if informative at all,
can be used to give evolutionary information of a type which no other morphological
detail can supply.
The comparison of chromosome numbers between related forms can in favourable
instances be a conclusive guide to phylogeny. The classic case of Spartina Townsendii
is a well-known example. This putative hybrid detected in Southampton Water in
1870,* and variously listed by Wallace, Hooker and others as an endemic variety or
species, or as an interspecific hybrid of spontaneous local origin, was conclusively
proved to be the cross between our native S. stricta and a locally introduced alien from
* A good general account of the history of the discovery and spread of S. Townsendii on the British
and French coasts will be found in Stapf (1927), from which it appears that the first person to propose a
hybrid origin for the species was a French botanist, Foucaud, in 1894. Other early references to the
plant are Wallace's Island Life, 2nd ed. (1892), and Hooker's Student's Flora. Huskins (1931) contains the
cytological facts and final diagnosis.
INTRODUCTION TO THE METHOD
North America, S. alterniflora, as soon as an accurate chromosome count could be made
(cf. Huskins, 1931) of the three forms in question. The endemic species S. Townsendii
could then be seen to contain twice the sum of the gametic chromosome numbers of
the two putative parent species and was therefore clearly the hybrid between those
species, which had doubled its chromosomes and become fertile and true breeding in
consequence.
Comparative morpholog}' of chromosomes may also give higlily significant information,
some very striking examples being the work of Babcock (1947) and his colleagues and
collaborators on Crepis in plants and the numerous studies on various genera of flies,
notably Drosophila and Sciara, some of which have been summarized recently by
Dobzhansky (1937) and by White (1945). From these it is clear how important a part
gross structural changes have played in the evolution of species in these groups.
A still more powerful tool lies in the accurate analysis of chromosome pairing, either
at meiosis or, in the special case of the Diptera, in the salivary glands. The detection
of failure of pairing or irregular pairing in a wild plant or animal will often reveal a
hybrid with certainty where other evidence is inconclusive or misleading. The classic
cases of Drosera obovata (Rosenberg, 1909), Rosa (Blackburn and Harrison, 192 1),
Hieracium (Rosenberg, 191 7), etc., are examples which could be indefinitely multiplied.
This type of inquiry has been of enormous benefit to pure taxonomy wherever natural
barriers between species have been obscured by the existence of unrecognized hybrid
forms, some of which, as in the case of Rosa, may have been perpetuated from remote
ages by the adoption of a non-sexual mode of reproduction. The presence of extra
chromosomes is revealed by the formation of multivalent groups. These may occur
singly as in the sex chromosomes oiRumex acetosa, in which the male plant has a trivalent
group in place of the bivalent pair of the female, in this case betokening perhaps no
more than the fragmentation of one sex chromosome. The presence of multivalent
groups on a considerable scale more commonly denotes the presence of duplicated sets
of chromosomes which constitutes the phenomenon of polyploidy.* This may be
autopolyploid, i.e. due to exact duplication of identical gametic sets with at most only a
minor amount of mutational difference of ' genie ' origin to distinguish them. Tetra-
ploid Biscutella laevigata (see below) in the Swiss and Austrian Alps is an example of this
* Polyploidy (Winkler, 1916) is the name given to the state of a cell, organism, or tissue, in which
the number of chromosomes contained in the nucleus is a simple multiple of the chromosome number
of some other, related, cell, organism or tissue. When the nuclear cycle of the higher plants was fiist
worked out at the beginning of the century the words haploid and diploid were introduced to designate
the reduced and unreduced nuclear states of the same organism. When it was later realized that ether
multiples of the basic gametic number could exist, the term haploid has in certain contexts given place
to monoploid, and additional terms such as triploid, tetraploid, pentaploid, hexaploid and so on have
had to be introduced to describe the various members of a series, which collectively represents the state
of polyploidy. The word heteroploid is sometimes used as a synonym for polyploid {e.g. by Sharp, 1934),
as also is euploid (Tackholm, 1922). Special types of polyploidy, namely, autopolyploidy and allopoly-
ploidy (Kihara and Ono, 1926), are explained in the text on pp. 8-9; these have recently been abbrevi-
ated to autoploidy and alloploidy (Clausen, Keck and Hiesey, 1945).
The opposite condition in which the nuclei of related cells differ by some quantity which is not that
of a simple multiple of a basic or monoploid set of chromosomes is designated aneuploidy (Tackholm,
1922) or dysploidy (see Clausen. Keck and Hiesey, 1945).
INTRODUCTION TO THE METHOD
kind. If, on the other hand, the sets are so different from each other that their chromo-
somes cannot pair together, mukivalents are not formed and we have allopolyploidy
as in Spartina Townsendii, or to quote a more recently analysed case. Nasturtium uni-
seriatum (cf. p. lo). In both types of polyploidy precise phylogenetic evidence can be
obtained if the polyploid forms are crossed with others of lower or higher valency and
Fig. I. Diploid form of Biscutella from central France {B. arvernensis Jord.) showing young fruits, from
a herbarium specimen collected in July, near Clermont Ferrand in the Auvergne. Natural size.
the chromosome pairing in such crosses analysed. This method can indeed be applied
to all species or strains closely enough related to be crossed at all, and detailed informa-
tion can then be compiled about the relative homologies of their respective chromosomes
which may give a surprising insight into past history. Occasionally some similar evi-
dence can be obtained more directly by inducing apogamy or parthenogenesis in a
normally sexual organism and observing the presence or absence of chromosome pairing
in the supposedly haploid set.
6
INTRODUCTION TO THE METHOD
All these methods and others will meet us in the chapters which follow, but in
order to give the reader some preliminary insight into the type of observations actually
involved, it may be helpful to reproduce some of the photographs used in the analyses
of Biscutella and Nasturtium listed above.
Biscutella laevigata L. is a fragrant little yellow-flowered member of the cabbage
family, well known to tourists in Switzerland, France, Germany and Spain on account
of its curiously shaped fruits (Fig. i), which the Latin name oi Biscutella compares to
two shields, although at least one popular name ('Brillenschoten' in German) makes
the comparison rather with a pair of spectacles. The species B. laevigata does not
occur in Britain, but a number of different strains of it are met with as limestone rock
plants in the lowlands or in subalpine meadows in the mountains of central and
southern Europe. It is, however, by no means uniform throughout its range. The
German and French lowland types have i8 chromosomes except in their pollen grains
and embryo sacs, where the reduced number of 9 is found. In Switzerland and Austria,
on the other hand, the plants all have twice as many chromosomes, 36 being found
in their roots and 18 in most of their reproductive cells. Both types are, however, still
interfertile, and hybrids between them, possessing the intermediate chromosome
number of 27, are spontaneously formed when suitable plants are grown together in a
garden. Fig. 2a-c shows the somatic chromosome numbers of these three types of
plants, which may be taken as the first illustrative example of a polyploid series. The
number 9 which they all share in varying degrees is the gametic number of the lowest
member. This gametic number, which is fundamental to the whole series, is con-
veniently designated the monoploid number, in respect of which the plant with 18
chromosomes (Fig. 2a) is diploid, the one with 27 (Fig. 2b) triploid and that with 36
(Fig. 2c) tetraploid.
Chromosome pairing at meiosis in diploid, triploid and tetraploid B. laevigata is
shown in Fig. 2d-f. In the diploid (Fig. 2d), pairing occurs in the simplest manner
possible and nine pairs can easily be seen in the photograph. In the tetraploid of
Fig. 2/, however, pairing is more complex, for the presence of four instead of two
monoploid sets of chromosomes has led to the formation of numerous quadrivalent
groups easily recognizable as such, where the four component chromosomes of a
quadrivalent are joined in a ring, as may be seen in many places to the right of the
figure. In the triploid (Fig. 2e), where there are three duplicate sets of chromosomes,
trivalents and not quadrivalents are formed. These are also easily recognizable by
their shapes, and in the cell figured there are five trivalents, four pairs and four uni-
valents, the pairs and univalents representing potential trivalents which have fallen
apart at an earlier stage into 2 + i . Falling apart into lower valency components is
liable always to affect a certain proportion of potential multivalent groups, since the
successful cohesion among a group depends not only on homology (that is ability to pair)
but also on the number and relative positions of the chiasmata which form after
pairing has taken place and by means of which cohesion up till metaphase is made
possible. Since the position of chiasmata is, to some extent, determined at random, the
precise numbers of effective multivalents which appear at metaphase will vary some-
what from cell to cell. The numbers of multivalent groups visible in Fig. 2e and /are,
INTRODUCTION TO THE METHOD
however, sufficiently high to be indicative of aMtopolyploidy in the cytological sense,
by which is meant the duphcation of identical, i.e. 'homologous', sets of chromo-
somes throughout the series, ahhough the existence of slight genetical differences
between the sets is not thereby excluded.
•4 '
0 '
• /
)
I ■
40
d - e f
Fig. 2. Polyploidy in Biscutella laevigata L. a-c from Manton (1935a); 6?-/ from Manton (1937).
a. A diploid root of subsp. alsatica showing 18 chromosomes, from a section stained in gentian
violet. X 2000. b. A triploid root of a hybrid between subsp. alsatica and subsp. longifolia showing
27 chromosomes, from a section stained in haematoxylin. x 2000. c. A tetraploid root of subsp.
longifolia showing 36 chromosomes, from a section stained in gentian violet, x 2000. d. Chromo-
some pairing in the diploid showing 9 pairs, from a permanent acetocarmine preparation, x 1500.
e. Chromosome pairing in the triploid showing trivalents, pairs and univalents, permanent aceto-
carmine. X 1500. /. Chromosome pairing in the tetraploid showing quadrivalents and pairs,
permanent acetocarmine. x 1000.
From the evolutionary point of view these facts show that the natural populations of
diploid and tetraploid B. laevigata in different parts of Europe are closely related
phyletically and that the diploids are the older type. The first of these conclusions
could have been deduced by the normal procedure of comparative morphology but the
second could not, and in this particular case the association of newer and older forms
with glaciated and unglaciated areas of central Europe respectively made it possible to
suggest some rough approximations regarding their relative ages and the paths of
8
INTRODUCTION TO THE METHOD
migration into their present habitats in terms of the Quaternary Ice Age (Manton,
1934, 1937)-
The case of watercress {Nasturtium) is sHghtly more intricate because the chromo-
somes are more numerous and smaller, and there are also a greater number of types of
plant to consider. A selection only of the relevant photographs is given in Fig. 3 a-/,
the magnification being the same as for Biscutella, but, to facilitate the interpretation in
view of their smaller size, some explanatory diagrams are added (Fig. /\.a-d). The
monoploid chromosome number (which is also the 'haploid' or gametic number of
the lowest form) is here 16, and the polyploid series again consists of diploids, triploids
and tetraploids which, in this case, possess chromosome numbers of 32, 48 and 64
respectively. Sample views of the somatic chromosomes showing the diploid and tetra-
ploid chromosome numbers in the unpaired state are contained in Fig. 3 a and b,
and the only further point of importance to add about the origin of the series is that
in this case all three members of it are wild plants widespread in Europe, although the
triploid, as in Biscutella, is the hybrid between diploid and tetraploid which, in the
watercress, has occurred spontaneously.
Chromosome pairing at meiosis in wild plants of diploid, triploid and tetraploid
watercress is shown in Fig. ^c-e, with an artificially produced autotetraploid (Fig. 3/),
obtained from the diploid by treatment with colchicine, added for comparison. In the
flw^otetraploid (Fig. 3/), chromosome pairing closely resembles that in tetraploid Biscu-
tella, allowance being made for the smaller size and greater total number of the chromo-
somes; numerous quadrivalents are formed. In the wild watercress polyploids, on the
other hand, multivalent groups are completely absent. The tetraploid (Figs. 3^, 4c)
forms 32 pairs and the triploid (Figs. 3^, ^.b) invariably develops 16 pairs and 16 uni-
valents, whether the plant studied be the wild triploid or an artificially synthesized
hybrid between the wild tetraploid and the diploid. This means that polyploidy in the
wild watercress cannot be simply due to the multiplication of identical sets of chromo-
somes. There must be two different sorts of monoploid sets contained in the polyploids,
one of which is identical with that of the low-numbered Nasturtium officinale R.Br.,
and which can pair readily with the chromosomes of that species when hybrids are
formed (as in the triploid) but the other of which is not homologous and which com-
pletely fails to pair in the triploid and does not form quadrivalents in the wild tetraploid.
The origin of this second set of 16 chromosomes is still unknown, though, from the
morphology of the fruits in the wild tetraploid, it is suspected to be a species oiCardamine.
The watercress series in the wild state is, therefore, not an autopolyploid series as in
Biscutella but an a//opolyploid one, and the wild tetraploid must be recognized as
in origin an interspecific, or in this case probably an intergeneric, hybrid since the
genus Nasturtium contains no other known species. This hybrid, at some former period,
doubled its chromosomes and became fertile and stable, as in the case of Spartina
Townsendii. It is therefore desirable in the case of watercress to separate the wild
tetraploid taxonomically from the diploid and to relegate it to a separate species to
which the name Nasturtium uniseriatum has been given as a descriptive title to record
the most distinctive morphological difference by which the tetraploid can be recognized
in the field without a chromosome count, namely, the arrangement of seeds in the
INTRODUCTION TO THE METHOD
fruit. The genus Nasturtium thus now contains not one single species as had previously
been thought but two separate species, one of which is older than the other, and partly,
though not wholly, parental to it. The two species from the taxonomist's point of view
A * ' iBBIm
• * 1
Fig. 3. Polyploidy in watercress [Nasturtium), a after Manton (1935), the rest after Howard and
Manton (1946). a. A diploid root in the true watercress [N. officinale R.Br.) showing 32 chromo-
somes, from a section stained in gentian violet, x 2000. b. Tetraploid cell in prophase from the
tapetum of a plant derived from the diploid by colchicine treatment showing 64 chromosomes,
permanent acetocarmine. x 1500. c. Chromosome pairing in the diploid showing 16 pairs,
permanent acetocarmine. x 1500. d. Chromosome pairing in the triploid hybrid between the
diploid and the wild tetraploid showing 16 pairs and 16 univalents, permanent acetocarmine.
X 1500. For explanatory diagram see Fig. ^b. e. Chromosome pairing in the wild tetraploid
(jV. uniseriatum How. & Mant.) showing 32 pairs, permanent acetocarmine. x 1500. For
explanatory diagram see Fig. 4c. /. Chromosome pairing in the autotetraploid of b showing
quadrivalents and pairs. For explanatory diagram see Fig. ^d.
are conveniently designated as JV. officinale R.Br, and N. uniseriatum Howard & Manton,
respectively.
Since JV. uniseriatum* (unlike Spartina Townsendii) has existed for long enough to
* Some important new facts have recently been added to this subject by H. K. A. Shaw (1947) from a
study of the literature and herbarium material which appears to show that the tetraploid species is
probably more widespread than had previously been thought since specimens attributed to the new
species have recently been reported from as far afield as America, Africa and Afghanistan. The name
10
■ INTRODUCTION TO THE METHOD
extend its geographical range throughout Great Britain and over a part at least of the
continent of Europe (and perhaps farther afield) it is possible that it may be on the
way towards the production of yet other new forms; for the triploid hybrid between
Nasturtium uniseriatum and JV. officinale, which is also widespread in Europe, is quite
capable of giving rise to a variety of new types by strictly cytological means. In any
case the triploid itself must be recognized as a type of watercress which is necessarily
still younger than its parent species and which is at present spreading vegetatively.
o J^
"o
•» * ^ ^ ;y "^'
^Srf^
4
4
^^-h
4
c
Fig. 4. Explanatory diagrams to Fig. 3. x 2000. After Howard and Manton (1946). a. Diploid
watercress (Fig. 3c). b. Triploid hybrid (Fig. -^d). c. Wild tetraploid [Nasturtium iiniseriatiim
How. & Mant.) (Fig. 3^). d. Autotetraploid watercress (Fig. 3/).
Looking again to the past, however, it may be suggested that the low-numbered
N. officinale itself may not be quite as simple as it seems, for a 'haploid' as high as n= 16
is unusual among its nearer relatives, the majority of which (in the genus Cardamine
and others) possess « = 8. It may therefore be suspected that Nasturtium officinale may
in turn have originated at some still earlier period and at a lower cytological level by a
repetition of the processes which have since produced N. uniseriatum. The last is specula-
jV. uniseriatum may therefore have to be superseded on grounds of priority by one of two other possible
synonyms. For the sake of simplicity these emendations have for the moment been disregarded, since
they are at present not based on the same type of cytological evidence as that quoted in the text,
which, for convenience, follows Howard and Manton (1946), though without prejudice to the ultimate
adoption of Shaw's proposals should they be further authenticated.
II
INTRODUCTION TO THE METHOD '
tion, but it is sufficient to indicate how a factual record of evolution within a small
group could be pieced together and to see how the periodic repetition of a few simple
processes such as hybridization and chromosome doubUng can give rise to a cluster
of species of different ages which, it may be said in passing, have no doubt been subjected
at every stage to natural selection but have not been caused by that process.
We could multiply examples, choosing other and better instances, but this is perhaps
enough to introduce the subject to an impatient reader who, at this point, may legiti-
mately be expected to ask 'What, after all, does this amount to? You have shown, it is
true, some facts about the mode of origin of a few rather trivial species and have traced
some events in their past history, extending some pakry thousands of years beyond our
little human lifetime, but is this Macroevolution?'
And this, of course, is the unsolved question. The interest of cytogenetic analysis
lies in the fact that it does indeed permit of some extension bacEwards in time beyond
our own very Hmited experience, and that it provides some phyletic information regard-
ing more important natural units than the artificially produced varieties of domesticated
animals and cultivated plants. But the gulf between this and macroevolution in the
literal sense is enormous. The tremendous changes recorded in the rocks and in the
taxonomic systems of the plant and animal kingdoms are so greatly different in degree
from anything which our existing analysis has so far touched that we cannot with
certainty know whether they may not also be different in kind. In that case we should
have in the end to admit that all our present tools can only touch the fringe of the
subject, and that such knowledge as we can acquire of the origin of species does not
necessarily provide the clue that we are seeking.
A decision as to this, however, may perhaps be left in the hands of our imaginary
historian who, from his vantage point in the twenty-first century or later, ought to be
able to see things in better perspective than is attainable by us. For ourselves we may
be satisfied with the knowledge that with some new tools in our hands we have a large
new field to explore, and if our exploration does not resolve the major problems of the
organic world, we may at least look forward to some enjoyable experiences which
may enhance our interest in some famihar and common plants.
12
CHAPTER 2
INTRODUCTION TO THE PROBLEM
The object of this book is nothing more ambitious than to assemble for the first time
some material for a prehminary comparison of evolutionary processes, as revealed by
cytology, in an ancient and a modern group of plants. The modern group which will
be kept particularly in mind, on grounds both of suitability and the accident of close
personal acquaintance, is the family of Flowering Plants, the Cruciferae. To this group
both the examples discussed in detail in the last chapter belong and a good deal of
other cytogenetic information is already available in published form. The ancient
group, as will already be obvious, is the assemblage of ferns and fern-hke plants known
as the Pteridophyta. For these, so little of the new knowledge has previously been
acquired, partly owing to technical difficulty, that the first purpose of the chapters
which follow will be to elicit the fundamental data.
The Crucifers are a family of about 220* genera and 1900* species of Dicotyledons,
neither outstandingly primitive nor specially advanced. They are common in Europe
but are spread to some extent the world over. Of fossil record they have little or none,
but they may reasonably be assumed to have arisen, or at least to have become estab-
lished, during the Tertiary period. That they are still actively evolving is suggested
first by the prolific development of new forms of domesticated species such as Brassica
oleracea, the Cabbage, and secondly by the relatively numerous cases of wild taxonomic
species of undoubtedly recent origin which cytogenetic analysis has already detected.
Many of the classic 'Jordanian' or ' microspecies ' of Erophila verna first studied in the
1850's by Alexis Jordan, and in modern times by Winge (1940), are certainly of this
nature, and other examples have already been mentioned in the preceding chapter.
That hybridization and polyploidy have been potent sources of species formation in
the family is known both from observations of the type already quoted for Nasturtium
and Biscutella and also from the beautiful experimental work of investigators such as
Karpechenko (1928 and earlier), who, in the famous case of Raphanobrassica, induced
the formation of closely comparable new forms artificially. That polyploidy itself has been
initiated repeatedly throughout the family is known from the simple evidence of com-
parative chromosome numbers among related species in many of the genera (cf.
Jaretzky, 1932; Manton, 1932 a). t Such simple comparison cannot diagnose the type
of polyploidy involved (whether auto- or alio-), nor determine whether the numerical
change preceded, followed, or caused the emergence of the species affected by it.
It does, however, show beyond dispute that polyploidy in some form or other has
entered into the evolutionary history of at least thirty out of the eighty-odd genera
examined. This is certainly an underestimate of its frequency, for in several genera
* Willis's Dictionary, 1925.
t See also general lists of chromosome numbers by Tischler, Gaiser, Darlington and Ammal, and
Maude.
13
INTRODUCTION TO THE PROBLEM
it must have occurred more than once, since different species are polyploid in diflferent
degree, while in others it may have so far escaped detection owing to an insufficient
number of species available for study. That a proportion, possibly a high proportion,
of these cases are allopolyploids of the type of Nasturtium uniseriatum seems highly
probable both from experience within the family (e.g. Erophila, Raphanobrassica, etc.)
and from the sum of evidence from other flowering plants.
There are, however, undoubtedly other methods of species formation in operation.
Thus, polyploidy apart, the diploid species oi Biscutella of the Laevigatae section appear at
present to be evolving by purely genetical means. Many of them have a distinctive
morphology, which is retained in cultivation, and a characteristic ecological habitat :
the high alpine B. mollis, endemic to Austria, and the French B. Lamottii confined to
extinct volcanic ash heaps of Tertiary age in the Auvergne, being good examples.
Some of the morphological and probably also physiological characteristics of these
species (or ecotypes as they may perhaps more properly be called) are undoubtedly
adaptive and enable their possessors to colonize successfully types of habitat which are
completely closed to the parental forms which, in both cases, are almost certainly still
present in the same geographical area though far removed from the actual sites. This
at once confronts us with something closely resembhng simple Darwinism, but the
species in question are all still sufficiently akin to be interfertile if brought artificially
into contact and have completely regular chromosome pairing in their hybrids.
Experience has, however, shown that speciation by genetical means uncomplicated
by other cytological changes is either relatively unusual or only a passing phase.
Sooner or later in most species, and probably also at some future time in Biscutella,
internal changes in the chromosomes set in so that homology is lost. Chromosome
pairing in hybrids then ceases to occur, and we have the type of sterility barrier which
is the taxonomist's greatest ally in determining natural specific boundaries. We know
very little, unfortunately, about the mechanisms by which sterility barriers due to a
loss of chromosome homology are brought about.* The pioneer in drawing attention
to the need for such knowledge was again Bateson in 191 3. A beginning has, however,
been made by cytogenetic work carried out largely in America, Russia, and to a lesser
extent in Great Britain, on a wide range of experimental plants and animals in which
the effects on chromosome pairing of visible lesions such as fragmentation, translocation,
segmental interchange, etc., have been studied. The exact role of these processes in
species formation is less easily demonstrated than are the other methods previously
discussed, though the work on the species of Drosophila and Crepis mentioned on p. 5 is
sufficient indication that in certain cases their importance is considerable. In the Cruci-
ferae there is practically no work directly devoted to such studies, but the mere observa-
tion that species hybrids here, as elsewhere, are more commonly sterile than fertile is
sufficient ground for believing that here also an important effect of intrachromosomal
structural changes must be envisaged.
A different type of nuclear upheaval to which the intrachromosomal changes just
mentioned may perhaps provide a clue, is that of aneuploidy (sometimes termed
* An outstanding exception is to be found in the work of Tobgy and Gerassimowa on Crepis, sum-
marized by Babcock ( 1 947 a) .
14
INTRODUCTION TO THE PROBLEM
dysploidy)* by which is meant a numerical change of some type other than that of a
mere mukipUcation of whole gametic sets. The genus Biscutella, from which so much
information has already been obtained, may again be quoted in illustration. In the
Laevigatae section one species of very limited geographical range in southern Spain has
a haploid chromosome number not of 9 but of 6. At first sight it might be suggested
that the numerical relation between 9 and 6 is such that both could be parts of a poly-
ploid series on 3. In this case, however, there is no direct evidence to support such a
contention and much which directly contradicts it. A gametic number of 3 is not
known in the Cruciferae at all, and the species in question has all the appearance of
being recent. Moreover, all other species of Biscutella in sections other than the
Laevigatae, together with species of a number of neighbouring genera, have a basic
haploid chromosome number not of 3, 6 or 9 but of 8. It therefore seems almost
certain that the Laevigatae section itself arose at some remote period, probably in the
Tertiary, by an aneuploid nuclear change which produced a numerical difference of
one chromosome in the basic haploid count. This change presumably at first was
associated with the production of one new species. This species must, however, have
been somewhat more different from its fellows than usual,! for when sufficient time
had elapsed for further new forms to develop from it in ways that we have recently
touched upon, the taxonomists studying the group with only morphology as a guide
were constrained to segregate them together into a separate section. In other cases,
no doubt, the size of the unit might have been not a section or subgenus but a genus
or larger group. Here, therefore, we are confronted suddenly with something very
Hke the old mutation theory in one of its simplest forms. And the least that we may
conclude is that new species are not all equally potent as evolving units, but that the
precise means by which they have come about may have a decisive influence on their
subsequent fate.
The difference between the relative importance of aneuploidy and polyploidy was
probably the most significant general conclusion which the work on the Cruciferae
brought out (cf. Manton, 19320), and the evidence can be repeated again and again
in other families. Polyploidy in the flowering plants abounds as a means of formation
of the type of species which do not fundamentally break new ground and the chromo-
some numbers reached maybe high; in the Cruciferae some of the highest known are
9n = 8i in nonaploid Biscutella and 8n= 120 in some varieties of Crambe, in which the
basic haploid number appears to be n=i^. The aneuploid changes, on the other
hand, are, with very few exceptions, characteristically associated not with species but
with groups of species, i.e. subgenera, genera, or larger units. They are far less frequent
* See footnote on p. 5 for further definition of terms.
t Very important recent evidence from Crepis summarized by Babcock (1947) indicates that some-
times the initiation of sterility barriers and morphological changes in the form of an organism may occur
in the reverse order to that postulated above, the sterility barrier associated with aneuploidy being
produced at first without external morphological changes which only ensue subsequently as a result
of the isolation imposed. If this can be shown to be the usual order, it will represent a considerable
advance in our knowledge of evolutionary mechanisms, though the essential point which is being made
above, namely, that the morphological changes ultimately associated with aneuploidy are of a more
far-reaching kind than those accompanying polyploidy, would remain unaffected.
15
INTRODUCTION TO THE PROBLEM
than the polyploid changes, and whilst clearly being more important, they have another
remarkable characteristic, namely, that the changes involved are always of a low
numerical order. The fundamental haploid numbers of genera known in the family are
limited to 5, 6, 7, 8, 9, 11 and 15, and though one might have expected that the poly-
ploid species could in turn have experienced comparable changes to give rise to new
genera with, say, basic haploid numbers of the order of 82 or 121, they do not, in actual
fact, appear to have done so. The two types of change would thus appear to be not
merely different in kind and in importance but in some sense, and for reasons which
are not at once self-evident, to be mutually incompatible.
Since macroevolution would seem at the very least to require the successive origin of
genera rather than the blind multiplication of species, this antagonism between two
different types of evolutionary processes was felt to be possibly a matter of considerable
importance. It was also a matter on which further hght might be expected to be
thrown if some comparative data relating to a longer period of evolutionary activity
could be obtained. This was the reason why attention was first directed to the
Pteridophyta.
In contrast to the Cruciferae, which may be regarded as a representative sample of
the dominant vegetation of to-day, all relatively new in a geological sense and still
in active development, the Pteridophyta are a mixture of newish forms such as some of
the leptosporangiate ferns, and forms of extreme antiquity, the last survivors of the
dominant vegetation of periods before the flowering plants existed. It is only necessary
to recall the obvious structural affinity between Equisetum and the Carboniferous
Calamites, or between the existing Lycopods and the fossil Lepidodendroids, or the still
more distant Devonian Drepanophycus and Silurian Baragwanathia ; or the very probable
affinity between the living Psilotales and the Devonian Psilophytales ; or between all the
ferns and the Carboniferous Coenopterideae, to realize that in this group we have a record
which cannot be equalled by that of any other types of living plants, and that they have
existed in some form since the earliest times from which the vegetation of the land has
been preserved.
Here, therefore, if anywhere, should the cumulative effects of different sorts of
evolutionary mechanisms, working over long periods of time, be discernible.
To the non-botanical reader, if such there be, to whom the above enumeration
provides merely a list of names, it may be helpful to mention briefly some of the
principal characteristics which unite an otherwise very varied collection of plants into
one great group. The members of the Pteridophyta are all vascular plants, possessing a
water-conducting system composed of characteristic lignified cells known as tracheids.
This lignified tissue differentiates them at once from the lower groups of land plants such
as the mosses or fungi and accounts for their relative ease of fossilization ; the tracheidal
structure is, however, a relatively primitive type of tissue, and is certainly more ancient
than the system of continuous tubes, the wood vessels, to be found in the Flowering Plants.
Seeds also are absent from the Pteridophyta, thereby distinguishing them from
both the higher groups of land plants which reproduce by seed, namely, the Gymno-
sperms and Flowering Plants. Reproduction in the Pteridophyta is by spores which are
liberated from sporangia borne on a variety of organs. They may be on the backs or
16
INTRODUCTION TO THE PROBLEM
edges of the leaves in ferns, on peculiar lateral appendages in the Psilotales, on the
upper surfaces of leaves in the Lycopods or attached to special members known as
' sporangiophores ' of possibly axial nature and aggregated into cones in the Equisetales;
lastly, in the ancient and undoubtedly primitive fossil Psilophytales the sporangia
terminate the ordinary, dichotomously branched, vegetative axes.
'Flowers' are absent, the relative inconspicuousness of the spore-bearing members
being associated with a complete absence of anything comparable to insect pollination.
Instead, the male sexual cells are themselves endowed with active powers of self-
motility, and these swimming spermatozoids (see Frontispiece) are the immediate means
by which fertilization is brought about. The dependence on free water which this
mechanism entails is thought to be a primitive feature, possibly inherited from an
algal ancestor and certainly shared by other lowly land plants such as the mosses and
liverworts; in the higher plants it has been lost.
The two sex organs which respectively either liberate the spermatozoids or retain
the egg during and after fertilization, are not borne on the same plant as the sporangia
referred to above. The sporangia liberate unicellular spores which germinate without
the intervention of any sexual process into little plants known as prothalli which are
always both smaller in size and surprisingly different in structure from the individual
from which the spores were derived. Unlike the parent plant a prothallus is never
differentiated into stem, leaves and roots and is devoid of vascular tissue (except in a
few abnormal cases). It is generally attached to the soil by delicate unicellular hairs,
the rhizoids, and it may or may not be green. Where the green pigment is present the
prothallus supports itself by the same physiological mechanism, involving photo-
synthesis, as the parent plant; this is the case in Equisetum, most ferns and some Lycopods.
Where chlorophyll is absent the prothallus may either live in symbiotic relation with a
fungus, as a saprophyte, a mode of life adopted by the prothalli of the Psilotales,
many Lycopods and the Ophioglossaceae among the ferns; or the prothallus may
be so short lived that it is able to complete its entire development on the food reserves
laid up for it in the original spore, this is the case in all the ' heterosporous ' members
of the Pteridophyta, namely, Isoetes, Selaginella and the Hydropterideae among the ferns.
Sooner or later, a prothallus of any of these varied types becomes sexually mature.
Antheridia are then produced, either superficially or sunk in the tissue, which will
liberate many swimming male gametes if water is provided at the right time. The
female gametes or eggs are large, non-motile cells, borne singly in characteristic female
sex organs, the archegonia. The basal part of an archegonium, containing the egg,
is sunk in the tissue of the prothallus which thereby adds to its protection, the rest of the
organ being in the form of a projecting multicellular neck which is capable of opening
when wetted, to give free access from the outside world to the naked surface of the egg
cell. When the union of the gametes has been achieved, it is immediately followed
by germination of the fertilized egg in situ. At first, it remains enclosed and nourished
by the prothallial tissue, but as soon as the multicellular embryo has become large
enough for the first organ (stem, leaf or root as the case may be) to assume its proper
function, the prothallial tissue is broken through and an independent organism with
the morphology of the spore-bearing plant is re-established.
MFC
17
INTRODUCTION TO THE PROBLEM
This type of life history in which sexual and asexual modes of reproduction are
separated on different individuals which succeed each other in regular order is known
as alternation of generations. In the animal kingdom such a life history is rare and
altogether absent from the higher forms, but in plants alternation of generations is so
deeply estabhshed in all the principal land plants except the fungi, that its effects are
still easily to be recognized even in the highest seed plants. In these, however, the
sexual, haploid generation or gametophyte is so much reduced that for many purposes
its existence may be ignored, and it commonly is so by geneticists. The flowering plants
Fig. 5. The sporophyte generation of a horsetail {Equisetum).
Willd. in the month of May. Natural size. b. Ripe spores of £. palustre L
a young sporangium of £. robustum A.Br, with meiosis in progress, x 100.
Sterile and fertile shoots of E. limosum
X 100. c. Section of
can then be treated exactly like the higher animals as diploid organisms in which the
nuclear reduction (meiosis) takes place as part of the maturation process of the gametes.
In the Pteridophyta the complete independence of the two generations for most of their
lives makes it necessary to retain the technically correct terminology appropriate to
botany or misunderstandings will follow. The conspicuous generation here (as tech-
nically also in the flowering plants) is a diploid organism reproducing by asexual
means and known in consequence as the sporophyte. The haploid, sexual plant (in the
case of the Pteridophyta the inconspicuous but free-living prothallus) is the gameto-
phyte. Sporophytes and gametophytes occur in all the higher groups of plants, though
18
INTRODUCTION TO THE PROBLEM
the special type of gametophyte to which the name prothallus is given is confined to the
Pteridophyta. In none of these cases is meiosis a maturation for the gametes, but it
Fig. 6. The gametophyte generation of a horsetail {Equisetum). a. Three prothalh of £. sylvaticum L.
of various ages mounted in glycerine jelly, x lo. The largest specimen bears a young sporophyte,
the uppermost specimen is a young female with a recently fertilized archegonium containing
an embryo, and the right-hand specimen is a male with numerous antheridia which appear dark
owing to density of contents, b. .\ntheridia of various ages from a whole mount stained in
braziline. x 200. Spermatozoids are being extruded from the right-hand antheridium, c. A
recently fertilized archegonium with several spermatozoids in contact with the egg cell, the
archegonial neck appearing closed and partly shrivelled, perhaps owing to the treatment, from
a section stained in haematoxylin, fixed within an hour of insemination, x 200.
takes an essential part in the formation of the asexual spores and in so doing forms the
point of demarcation between sporophyte and gametophyte, the other point of demarca-
tion (between gametophyte and sporophyte) being the syngamous nuclear fusion. Some
19
2-2
INTRODUCTION TO THE PROBLEM
of these facts may perhaps be helpfully summed up by a glance at Figs. 5 and 6, which
show characteristic views of both generations, together with their respective repro-
ductive organs for one particular member of the Pteridophyta, the Horsetail [Equisetum) .
All this will be so familiar as to be second nature to a botanical reader; nevertheless,
even for such, it may perhaps be convenient and save risk of confusion if two other
Fig. 7. Some examples of apogamy in ferns, a. Obligate (or 'direct') apogamy in i)rvo/;ferw5orrmNewni.
' war . polydactyla Wills ' (for further details see Chapters 5 and 1 1 ) showing a young apogamous outgrowth
from the region of the prothallus where the central cushion bearing archegonia should be. x lo.
b. Induced apogamy in an abnormal prothallus of Osmunda regalis L. derived from a spore produced
by the triploid (see Chapter 3) and too unbalanced genetically to be able to reproduce normally;
after several years' sterility a leaf is being produced from the apex of the prothalloid tissue, but so
far the specimen has no roots, x 2. c. The same as b in other examples. Natural size.
terms, without exact zoological equivalents, be referred to at this point. 'Apogamy'
and 'apospory' are two aberrations of life history by which the regular sequence of
sexual and asexual reproductionjust explained is modified. 'Apogamy' is the production
of a sporophyte from a gametophyte without the intervention of a sexual process. A
special case of apogamy is 'parthenogenesis', a name used by both botanists and
zoologists to denote the development of an egg without the act of fertilization. This is
unknown in the Pteridophyta, although it undoubtedly occurs under exceptional
20
INTRODUCTION TO THE PROBLEM
circumstances in the Flowering Plants and is not uncommon in the Thallophyta. In
other groups, if fertilization is prevented, the egg itself invariably dies, but other parts
of the gametophyte may, under suitable conditions, proliferate directly into sporophytic
tissue. Sometimes, through a genetical aberration, such proliferation may become the
rule, and one or both of the normal sex organs may become eliminated from the life
history in consequence; several examples of this in ferns will be examined cytologically
below. In other cases a genetically normal prothallus can, by prolonged prevention of
fertilization through the withholding of free water, be induced to develop apogamously
as an exceptional circumstance; some examples of this will also be examined. In both
a ~ b
Fig. 8. Some examples of apospory in ferns, a. Apospory induced by malnutrition in an otherwdse
normal young plant of Osmunda regalis L. var. cristata Moore; the petiole of a juvenile leaf, shown at
the base of the specimen, after forking twice has grown out into a forked prothallus covered with
rhizoids. From a photograph supplied by Prof. W. H. Lang, x 8 approximately, b. Apospory
in Athyrium Filix-femina (L.) Roth var. clarissima ]one.s. Portions of an adult frond laid on soil and
kept moist for several weeks have proliferated into prothalli, some of which are already bearing
apogamous young plants. Natural size.
cases the result is the same, a sporophyte with the gametophytic chromosome number
is produced.
'Apospory' is the aberration on the part of a sporophyte by which proliferation of
gametophytic tissue takes place from it without the intervention of normally con-
structed spores. As in the case of apogamy, it is sometimes possible to induce apospory
in genetically normal plants by appropriate experimental treatment. Cutting off the
leaves of juvenile plants and laying them on soil is a well-known method which, if
successful, will result in the proliferation of prothalli from the tips of the leaf or from its
cut surface. In other cases, owing to genetical peculiarity, the same process can be
carried out even with the large leaves of fully mature plants ; examples of this are
certain well-known horticultural 'varieties' such as 'Lastrea pseudo-mas' var. cristata-
21
INTRODUCTION TO THE PROBLEM
apospora Cropper or ' Asplenium Filix-foemina var. clarissima ]onts\ In all these cases,
whether genetically normal or not, the result of the aposporous development is a
prothallus with the same chromosome number as the parent sporophyte. Such pro-
thalh, in the case of the horticultural varieties, are generally apogamous, and nuclear
change in them is ehminated from the life cycle. Where apospory has been induced in
genetically normal plants, the sexual function of the induced prothalh may be un-
impaired; the consequence of this is a polyploid series.
A special case of abnormal life history to which the name apospory cannot strictly be
applied is sometimes found in association with permanent apogamy in ferns. Pteris
cretica and Cyrtomium falcatum are the classic examples, and in these the morphology of
the spores is retained, but the meiotic process is rendered ineffectual by a sequence of
unusual and highly interesting cytological happenings in the sporangium. Spores with
the unreduced chromosome number are formed, and these give rise to apogamously
reproducing prothalli. Again the nuclear basis for aUernation of generations has been
eliminated.
Some photographs in illustration of the phenomena of apogamy and apospory are
appended in Figs. 7 and 8, and examples of all these types of abnormal life history will
be examined below. Some of them are very instructive from an evolutionary point of
view.
With this amount of introduction we may turn to the plants themselves. Fig. 9 is a
reproduction, translated into English where necessary, of a fairly recent summary
of the probable relationships of the main living and fossil types. It is not necessary to
analyse it in detail,, for certain obvious general conclusions can be obtained at a glance.
The preponderance of extinct compared with living forms is striking and would doubt-
less be still more marked if account could be taken of extinct forms which have perished
without trace. It is also obvious that, in the living forms, we are dealing with the end-
products of phyletic lines which have long been separated. Of their somewhat im-
poverished modern representatives only the ferns can compare at all favourably with the
weahh of the known or presumed ancestral types, and the ferns {'Pteropsida') enormously
outnumber all the other living representatives added together.
The living members of the Pteridophyta may be hsted in the somewhat more
famihar terminology adopted by Bower as follows :
PsiLOTALEs: two living genera, Psilotum and Tmesipteris, both mainly tropical or sub-
tropical, each with one, or at most two, species.
Lygopodiales (Clubmosses) : four living genera. Lycopodium, world-wide distribution,
about 185* species. Phylloglossum confined to Australia and New Zealand, one
species. Selaginella, world-wide distribution, over 700 species. Isoetes, world-wide
distribution, 50 species.
Eq,uisetales (Horsetails) : one living genus, Equisetum, world-wide distribution, except
Australasia, about 25 species.
* These numbers and the following are quoted from Willis's Dictionary (1925).
22
CO
CO
c
m
B
u
N
u
u
c
-s
o
O
>^
r-*
bo
_c
CL,
CI
bb
INTRODUCTION TO THE PROBLEM
FiLiCALES (Ferns): about 150 genera and 6000* species grouped into orders and
families, of which the Ophioglossaceae, Hymenophyllaceae, Osmundaceae, Polypo-
diaceae and Marsiliaceae occur in Britain.
Some idea of the taxonomic and phyletic complexity of the ferns may be obtained
by a glance at Fig. 10 reproduced from Bower (1935). It is obvious that the ferns alone
could easily provide material for cytological study for more than a lifetime, and one of
the consequences of this book may perhaps be to suggest that such a study would be
Davalho/ds
Dickson laceoe
^ Pteroids
^ Osmundacece
%
P/ag/o^yriaceoe Gymnogrammoids
Cyadieacece
B/echnoids
•s
\
\
Dryoptero/ds
~ Dipf-eroids
Fig. 10. Phylogeny of the ferns, redrawn after Bower (1935).
worth pursuing. The little that is presented here in the first few chapters may, how-
ever, also supply a wholesome warning. The ground must be conquered step by step
with the utmost labour, and a rapid attack on all the intricacies of phylogeny cannot be
hoped for. It will therefore not be attempted at this stage except incidentally and
in small details, and attention will primarily be directed to those larger general questions
for which the smaller but perhaps equally ancient or older living groups are as important
as the ferns.
* These numbers are taken from Verdoorn's Manual of Pteridolog}', in which Christensen (1906) is
quoted. Since then Christensen (Supplement 1934) emended these figures to 213 genera and 9387
species. Other estimates will be found in Copeland (1947).
24
INTRODUCTION TO THE PROBLEM
These general questions are, first, to determine whether any of the evolutionary
mechanisms shown to be operating in the Cruciferae can also be detected in the
Pteridophyta. If this is answered in the affirmative the second question is to inquire
whether any further light can be thrown upon such mechanisms by observing the
accumulation of their effects in a longer period of geological time than the Flowering
Plants have yet had at their disposal. In precise terms this amounts to asking, in the
first place, whether the processes of genie mutation, aneuploidy, polyploidy, hybridiza-
tion and the rest, have or have not played a recognizable part in species formation in
the Pteridophyta; and secondly, to see if any of the predictions aroused by the data
already obtained from the Cruciferae can be confirmed or extended by comparison
with the older group. Consideration of this second question will be left to the end of the
book. The earlier chapters will primarily be devoted to ascertaining the facts relevant
to the first, and since the particular evolutionary processes of polyploidy and hybridiza-
tion are the easiest to demonstrate, these will be uppermost in mind in the early stages
of the inquiry.
25
CHAPTER 3
THE POLYPLOID SERIES IN OSMUNDA
Osmunda regalis, the Royal Fern,* is such a dehghtful cytological object that it may be
well to prepare the way for the evolutionary analysis which follows by devoting a pre-
liminary chapter to it. This will enable us to amplify the brief introduction to poly-
ploidy given in Chapter i and thereby to set up a standard of reference from within
the group itself, against which the facts for other plants of unknown origin can be
assessed.
The reasons for the selection of Osmunda are well known. It is easily grown, indeed
it is remarkably hard to kill, even in the centre of an industrial city. Both generations
can be treated as perennials. The spores are produced in abundance and will germinate
at once if scattered on to soil or tap water. The chromosomes are large and few in
number {n = 22, one of the lowest known numbers in the ferns) and they are readily
accessible, for both roots and sporangia are freely exposed in quantity with a minimum
of protective covering. Fixation, moreover, with modern fixatives presents no difficulty,
and all the more important special methods of treatment, including that for spiral struc-
ture (Manton, 1939), can be applied with success. Finally, in its morphology, the
sporophyte at least has shown itself to be remarkably plastic under experimental
treatment.
The origin of the autopolyploid series was apospory induced experimentally in a
normal strain of the species. The basic facts of its production were described in some
detail by Lang in 1924, and the cytology recorded in a preliminary form by Manton
in 1932. Briefly, the process consisted in the persistent depauperation of young plants
by mutilation and malnutrition, either deliberate or accidental, enhanced by prolonged
neglect during the war of 19 14-18. As a result, the normal adult type of foHage failed
to develop, and a variety of abnormalities affected the younger leaves which had other-
wise reverted to the juvenile type. Examples of some of these unusual growths are
shown in Fig. 1 1 a-d, which are reproduced from Lang's original drawings, and in
Fig. 12, which is one of Lang's original photographs. They represent various types of
cyhndrical processes, vegetative buds and prothalli which had developed from, or re-
placed, the leaf lamina or petiole. Prothalli were somewhat infrequent but were detected
more than once (see also Fig. 8a, Chapter 2). Such prothalli could produce rhizoids
and some sex organs whilst still attached to the parent plant or, if removed and laid on
soil, they grew on as normal-looking gametophytes and gave rise to numerous young
plants. Owing to their perennial habit the prothallial cultures could be kept growing
indefinitely and they are now over 20 years old and still continue to produce young
plants. The prothalli are, however, diploid and the young plants tetraploid,
* Anyone unfamiliar with the general appearance of this large and characteristic fern will find some
leaves of a smaller species illustrated in Chapter 16, Fig. 276a, ^, p. 275.
26
Fig. 1 1. LeafabnormalitiesinstarvedandmutiIatedyoungplantsofOj7n«««/are|^a//^L., after Lang (1924).
a. Part of an uninjured but starved specimen showing a bud {h) on one of the petioles, x 6.
h. The bud of a more highly magnified, x 20. c. One leaf from a similar plant showing more
extreme modification; there is no lamina, but instead the petiole forks and one fork bears a pro-
thallus [p) at its tip and a bud laterally, x 7. d. Whole plant found detached and without roots
in a crowded pan. x 6. Of the two expanded leaves one is normal and the other bears five
prothalli at the tips of the veins. This leaf, after being detached and laid on soil, was the start of the
polyploid series in Osmunda. For other examples of apospory of this type see the photographs in
Figs. 8a and 12.
27
THE POLYPLOID SERIES IN OSMUNDA
Since 1932 apospory of similar type has occurred spontaneously on several occasions
in young tetraploid sporophytes. The resulting prothalli are also tetraploid, but,
unlike the diploids, they appear to be sterile, for they are now over 10 years old and
are still barren in spite of liberal watering.
The triploid sporophytes and gametophytes, which complete the series, have required
somewhat more careful preparation. The first triploid sporophytes were already
fully grown and fertile when their chromosomes were first examined in 1932. They
occurred intermixed with the tetraploids which had been grown on from the first
experiments, and two were found among seven plants. Their origin was at that time
Fig. 12.
Fig. 13-
Fig. 12. The origin of the polyploid prothalli. Two 'leaves' from a young plant of Osmiinda regalis L.,
the one on the left showing the normal juvenile lamina, but the one on the right abnormal in shape
and ending in a heart-shaped prothallus. From a photograph kindly supplied by Prof. W. H. Lang,
after Lang (1924). Twice natural size.
Fig. 13. The origin of the polyploid sporophytes. Haploid and diploid prothalli fertilized on the same
day and grown on together. The normal (diploid) young plants on the right have grown much
faster than the tetraploid young plants on the left which therefore appear much smaller. Several
unfertilized prothalU can be seen creeping over the soil in the left half of the pot. Half natural size.
uncertain but has since been traced to contamination of the original diploid prothalhal
cultures with some normal prothalh from self-sown spores. Triploids have been
synthesized on several occasions since by inseminating diploid archegonia with
haploid spermatozoids, and they are not produced if the diploid prothallial cuhures
are kept pure. Though more troublesome to produce in large numbers than are the
tetraploids, such young triploid plants can be made to become aposporous in the
same way as the others. The method adopted by Mr Ashby, who has been entirely
responsible for this part of the work, was to mutilate a young triploid plant by repeatedly
removing its roots until the depauperate condition associated with leaf abnormahties
was achieved. One aposporous prothallus was developed which has since been grown
28
THE POLYPLOID SERIES IN OSMUNDA
on and subdivided. The triploid prothallial culture so obtained is now about 6 years
old. It has not yet produced a young plant, but it is perhaps still possible that in time
it may do so. The parent plant has been allowed to return to normal.
a be
Fig. 14. Comparative leaves from the polyploid series of Osmunda; terminal leaflets from sterile fronds
of comparable stature, a, diploid; b, triploid; c, tetraploid. The triploid shows gigantism, but the
tetraploid is depauperate and also abnormal in shape. Half natural size.
a b c d
Fig. 15. The polyploid prothalli of Osmunda. Natural size, photographed at the end of the growing
season, in autumn, a, b and d photographed at the same time under strictly comparable conditions ;
c from a different year, a, haploid; b, diploid; c, triploid; d, tetraploid. The diploid and triploid
both seem to show gigantism, but the tetraploid is small and abnormally indented at the edge.
Triploid and tetraploid are unable to reproduce, but haploid and diploid are very fertile.
While there is still some chance that the series may be extended in the future should
either of the high-numbered prothalli become fertile, until they do so further progress is
blocked. We must therefore be content with diploid, triploid and tetraploid sporo-
phytes and haploid, diploid, triploid and tetraploid gametophytes.
29
C^'
4
c d
Fig. 1 6. Comparative series of cell sizes, especially rhizoid diameters in the polyploid
prothalli of Osmunda. x 50. a, haploid; b, diploid; c, triploid; d, tetraploid.
30
THE POLYPLOID SERIES IN OSMUNDA
Some idea of the gross morphology of the various members of both generations
may be obtained from Figs. 14 and 15. Each member of the series, even a sterile pro-
thallus, is a complete individual provided with all the appropriate vegetative and
reproductive organs, though perhaps the female sex organs may not always be struc-
turally perfect. Differences in size, shape, growth rate and no doubt other physiological
processes nevertheless accompany polyploidy.
The first increase in chromosome number produces gigantism; this is conspicuously
the case for the triploid sporophytes and the diploid gametophytes, both of which are
larger than the corresponding normal individuals. With further increase of chromo-
some number size is again reduced. The triploid prothalli are almost as large as
Fig. 17. Comparative sizes of antheridia and spermatozoids in tetraploid (a)
and haploid {b) prothalli of Osmunda. x 1000.
the diploid though not quite so regular in appearance, but the tetraploids are smaller
even than the haploids, and the tetraploid sporophytes are depauperate in a similar
degree.
The high-numbered forms are not only depauperate but show structural irregularities.
Thus the tetraploid prothalli display a highly characteristic fimbriation of the margin,
which is foreshadowed to a less extent in the triploids. The equivalent effect in the
tetraploid sporophytes appears as an irregular lobing of the leaf.
These irregularities are likely, in part at least, to be the expression of increased cell
size coupled with reduced growth rate. Cell size increases continuously, throughout
the polyploid series. Fig. 16 shows the series of rhizoid diameters for the four types of
prothalli and Fig. 1 7 gives the extreme spermatozoid sizes. The large spermatozoids of
the tetraploid gametophytes have been seen to swim though with a slow and clumsy
motion; the reason for the sterility of these prothalli is therefore not obvious. Similar
31
^. "'l^"'
9
1
Fig. 1 8. Comparative sizes of sporangia in OjOTM«</a. x 5. a, diploid;
b, triploid; c, tetraploid.
Fig. 19. A cell at the beginning of meiosis at the stage known as leptotene (literally the thin thread
stage) before chromosome pairing has occurred. One complete, unpaired, chromosome has been
separated from the tangled mass of the others at the top of the field. Permanent acetocarmine
preparation of triploid Osmunda. x 2000. After Manton (1939).
32
THE POLYPLOID SERIES IN OSMUNDA
size relations can be demonstrated for the sporophytes by using the spore mother cells
or the epidermis from comparable regions of the leaf.
Increasing cell size is naturally reflected in increased size of particular organs
wherever these depend for their structure on a rather precise cellular arrangement.
This is particularly true of the reproductive organs, and gigantism continues to be
expressed in the antheridia and sporangia long after it has ceased to appear in the
total stature of the plant. This has already been seen in Fig. 1 7 for the antheridia, and a
comparable set of sporangia is shown in Fig. 18.
Table i . Comparative measurements of cells and organs in the autopolyploid
series q/Osmunda
Tissue
Observation
n
2«
3«
4«
Gametophyte :
Spore
Rhizoid
Spermatocytes
Antheridia
Diameter, alive (/x)
Diameter, alive (^)
Diameter, in section (/a)
Diameter, alive (/li)
80
18
5-5
60
90
26
7
80
33
•
42
9
160
Sporophyte :
Sporangia
Leaf
Diameter, alive (mm.)
Maximum length (cm.)
(under comparable conditions.
1946)
•
•
0-55
114
07
130
0-8
91
Reduction of growth rate is less easy to express in precise terms than are structural
characters. An ocular demonstration of it is, however, provided by Fig. 13. This shows
the relative speed of development of sexually produced offspring from haploid and
from diploid prothalli which were fertilized on the same day and grown on together
in one pot. The normals have far outstripped the polyploids. Another feature deter-
mined by growth rate is the date of shedding of spores in spring. Under identical
conditions of culture, dehiscence of sporangia in diploids and triploids occurs almost
simultaneously, with the diploids not more than a day or two in advance of the
triploids ; the tetraploids, in contrast, are always about a fortnight later.
For convenience of reference some of these observations are summarized in Table i,
and while it is obvious that many more could be made, especially with regard to the
comparative study of physiological processes, enough has perhaps been given to
provide a background to the cytological behaviour which, from the present point of
view, is the centre of interest.
The chromosomes of Osmunda are fortunately sufficiently large to provide an almost
diagrammatic demonstration of all the more important cytological manifestations of
autopolyploidy. The most important of these, for reasons which have already been
partly explained in Chapter i, is multivalent pairing at meiosis. In a normal diploid,
where only two sets of homologous chromosomes are present, the early stages of the
reduction process (prophases* of the first meiotic division) consist in the pairing
* To those unfamiliar with cytological nomenclature it may be helpful to explain that the words
prophase, metaphase, anaphase and telophase are the names given to successive stages of all types of cell division
whether mitotic or meiotic. Prophases are the early stages, metaphase is the equatorial plate stage when
MFC
33
THE POLYPLOID SERIES IN OSMUNDA
together laterally of every chromosome with the exactly equivalem homologous partner.
Whilst this is occurring the chromosomes are very long and thin, as may be seen by a
glance at Figs. 19 and 20, but when pairing is complete they appear to shorten and
thicken in a most remarkable way (cf. Fig. 22). This change of shape is partly due to
genuine shrinkage but is chiefly caused by the fact that each chromosome becomes
coiled into a spiral configuration (Fig. 21) which is commonly referred to as spiral
Q« ■■HP
Fig. 20. Paired chromosomes at the stage known as pachytene (literally the fat thread stage), a, a com-
plete bivalent in the diploid; b, a trivalent in the triploid; c, a quadrivalent in the tetraploid.
Permanent acetocarmine preparations, x 2000. a after Manton (1939); b after Manton (1945).
Structure but which can only be clearly seen after special treatment and which was
therefore for long disregarded. It will not often be necessary to refer to spiral structure
in the course of this book, though it is perhaps worth mention at this point, since
otherwise the changes of shape which affect a chromosome during prophase appear
unnecessarily mysterious.
the chromosomes are assembled on the spindle, anaphase is the stage at which movement to the poles takes
place, and telophase denotes the stage at which resting daughter nuclei are being reformed. It is not
customary to subdivide these stages further for mitotic divisions, but the prophases of meiosis are so
complex that a whole sequence of substages have been recognized and given separate names. The most
important of these are leptotene, zygotene, pachytene, diplotene and diakinesis, after which metaphase of the
first meiotic division sets in. Detailed description of these stages will be found in many elementary text-
books of cytology or genetics, but for the purpose of this book the descriptive notes given in this chapter
for leptotene, pachytene and diakinesis should suffice. The only other cytological nomenclature which might
perhaps cause confusion to a non-cytological reader concerns the appellations of the two meiotic divisions.
In the older literature the names heterotype and homotype were applied to the two nuclear divisions which
constitute meiosis ; these words are, however, becoming superseded by the rather simpler designation of
'first meiotic division' and 'second meiotic division', and the latter practice will be adhered to through-
out this book. The numerical chromosome reduction by means of chromosome pairing occurs at the first
division. The second division involves the longitudinal separation of half-chromosomes as in a somatic
mitosis. The result of the two divisions is a tetrad of four nuclei each with the reduced chromosome
number.
34
THE POLYPLOID SERIES IN OSMUNDA
Since the spiral forms independently in each chromosome the paired partners
lose contact with one another as prophase advances, except at a few places at
which an especially intimate relation has been established. Such places are termed
chiasmata and one, two or three may occur in any chromosome pair (in other
organisms with longer chromosomes the number may be greater) in positions which
are to some extent determined at random. The shapes of paired chromosomes are
affected by this, although the nature of chiasmata are not otherwise important for
our present purpose and need not therefore be discussed. If a pair of chromosomes
remains joined by a single chiasma it will look like a rod, V or X, according to whether
the chiasma is median in position or terminal. If there are two chiasmata a pair of
mm iPk "^
Fig. 21. The equatorial plate stage of the first meiotic division in diploid Osmunda showing 22 pairs
of chromosomes in which spiral structure has been revealed by ammonia treatment before fixation.
The formation of the spiral explains most, though not all, of the apparent changes of size and
shape which a chromosome experiences during prophase. Fresh acetocarmine. x 2000.
chromosomes will resemble an O or an a; if there are three it will look like a figure of
eight. Several of these shapes can be seen in Fig. 22 a, which represents the end of the
first meiotic prophase at the stage known as diakinesis. After this the paired chromo-
somes assemble on the spindle and then separate to opposite poles.
The details of the later stages of meiosis need not at the moment concern us, and it
is perhaps sufficient to refer to the anaphase of the second division (Fig. 22b) in which
the reduced (haploid or monoploid) number of 22 single chromosomes is very clearly
displayed.
When more than two homologous sets of chromosomes are present both meiotic
divisions are affected in a characteristic way. If there are three homologues of every
chromosome they will attempt to unite in threes, an attempt which is not always
35 . 3-'
THE POLYPLOID SERIES IN OSMUNDA
successful, in which case a potential trivalent will be represented by a pair and a
univalent. The reason for this may partly be seen by examining pachytene. A potential
trivalent in the fully extended condition is shown in Fig. 20b, and it looks rather like a
ArJB>" V»-
% J» ^ ^% ^ *^ ^
Fig. 22. Some other stages of meiosis in diploid and polyploid Osmunda. Permanent acetocarmine.
X 1000. a. Diakinesis (late prophase) in the diploid showing 22 pairs resembling those of Fig. 21
except that the spiral is not visible, b. The end of the second meiotic division showing two of
the daughter nuclei each with 22 single chromosomes, after Manton (1939). c. Diakinesis in
triploid Osmunda showing trivalents, pairs and univalents. For explanatory diagram see Fig. 23.
d. Diakinesis in tetraploid Osmunda showing quadrivalents and pairs. For explanatory diagram
see Fig. 24.
pair with the third chromosome wound round it. Pairing is, in fact, only possible
between two chromosomes at any one point, no matter how many homologues may be
present, and the only effect of the presence of additional potential partners is that
the identity of those in contact changes from time to time. Such changes of partner,
36
THE POLYPLOID SERIES IN OSMUNDA
if they occur in suitable positions for chiasmata to form in relation to each of the
homologues, will result in the estabUshment of a union between all three which is
retained until metaphase with the production of the characteristic shapes associated
with trivalents (Fig. 22c). If the chiasmata are unsuitably placed the group will fall
apart.
The analysis of the triploid nucleus of Fig. 22 c is given diagrammatically in Fig. 23,
in which trivalents are shown in black and pairs and univalents in outline. Similar
analyses for 109 cells are summarized in Table 2, and it is important to notice that
although every chromosome is known to be present in triplicate, a circumstance which
might lead one to expect that 22 trivalents would always be formed, the actual numbers
1^
«•
ifi
^
Q
^
Triploid Osmunda 3n - 66
Fig. 23. Explanatory diagram to Fig. 22c
with trivalents in black but pairs and
univalents in outline.
^@
ea
4^ ^^ -to
85
Tetraploid Osmunda -^/7 = 88
Fig. 24. Explanatory diagram to Fig. 22 rf
showing quadrivalents in black and pairs in
outline.
found show a random distribution round an ill-defined mean which is considerably
less than this (i.e. between 14 and 19). The observations recorded in the table are from
several plants in different years, a fact to which the absence of a well-defined peak in
the distribution is probably due, since the average almost certainly varies slightly
with external conditions from year to year. Had a larger sample been analysed,
the range of actual numbers would probably have been slightly wider, and it is
possible that in extremely rare cases the maximum of 22 trivalents are actually formed.
Such further possibilities are, however, of no importance from the present standpoint,
for the main purpose of Table 2 is to indicate that, had the origin and nature of the
plants been unknown, it would have been safe to infer autopolyploidy if the average
number of trivalents found could be shown to involve more than half the chromosomes
present. This fact has, of course, already been used for the analysis of the Biscutella
series noted in Chapter i .
37
THE POLYPLOID SERIES IN OSMUNDA
Table 2. Frequency of trivalents in 109 cells of autotriploid Osmunda (^2 = 22)
No. of trivalents per cell
7 8 9 10 II 12 13 14 15 16 17 18 19 20 21 22
No. of cells .11 I 5 7 9 II 13 12 12 17 12 8 .
Comparable information for the tetraploid is contained in Figs. 20c, 22^, 24 and in
Table 3. Where four homologous sets of chromosomes are seeking partners at meiosis,
some quadrivalents will inevitably result, though in a proportion of cases, the exact
numbers varying from cell to cell, a quadrivalent will be represented by two pairs or,
in a smaller proportion of cases, by a trivalent and a single. In the figured cell (Figs.
22^, 24) only pairs and quadrivalents are contained; this is a fairly common condition,
but examples with one or two trivalents and singles in addition to quadrivalents are
almost equally so. Table 4 gives the actual frequency of trivalents in the analysed
cells of Table 3.
Table 3. Analysis of multivalent pairing in 101 cells of autotetraploid Osmunda
No. per cell
■1
7
8
9
10
II
12
13
14
15
lb
17
18
19
20
21
22
3
I
4
10
II
12
14
13
13
9
8
I
I
.
1
.
.
.
2
I
12
8
16
15
15
15
12
2
I
I
I
.
Quadrivalents only
Total multivalents (quadri- + trivalents)
Table 4. Frequency of trivalents in 101 cells of autotetraploid Osmunda
No. per cell
, * ■ .
01234567
1 01 cells of Table 3 34 36 24 4 i 2
These numerical facts have been given, in what at first sight may seem to be rather
pedantic detail, for two reasons. In the first place they are needed to illustrate the
type of observation which, when it can be carried out, is more informative than any
other for cytogenetic analysis. Secondly, they are required to explain the breeding
behaviour of the polyploids, consideration of which will conclude this chapter.
The fertility and subsequent behaviour of the spores produced by any plant are
very closely connected with their nuclear content which, in its turn, is largely controlled
by the details of chromosome pairing at meiosis. Quadrivalents and bivalents can
disjoin regularly with equal ease, and if only these were produced, meiosis in a tetra-
ploid would be as uniform and effective as in a diploid. Trivalents and univalents
cannot, however, disjoin equally. From a trivalent two chromosomes will generally go
to one pole and one to the other at anaphase of the first meiotic division, so that the
resulting nuclei will at once be dissimilar. A univalent cannot disjoin at all, but instead
it lags on the spindle until it splits longitudinally. The half-chromosomes pass to each
pole at the close of the first meiotic division, but they lag again at the second, being
unable to split a second time. Finally, they either pass at random to one pole or the
other or are lost. Two views of the behaviour of univalents are given in Fig. 25.
38
THE POLYPLOID SERIES IN OSMUNDA
While the presence of both trivalents and univalents thus entails an element of random
variation in chromosome numbers at the close of meiosis, this is of only limited
extent in the tetraploid. In the triploid, on the other hand, variation will be extreme,
since random segregation here concerns not one or two chromosomes only but the
whole of the third haploid set. The probability that these will distribute themselves
to the poles to give exactly balanced spores with n or 2n chromosomes is only one in
about two million. All other types of spore will be unbalanced and possess chromo-
some numbers ranging from haploid to diploid but with the majority midway
between.
Fig. 25. The behaviour of univalents at the two meiotic divisions in triploid Osmunda. a. Anaphase
of the first meiotic division showing lagging univalents in the act of splitting longitudinally.
b. Anaphase of the second meiotic division showing the half chromosomes produced from univalents
at the previous division lagging again on the spindle in both dividing cells since they are unable to
split longitudinally a second time. Lagging chromosomes at either of these divisions are therefore
very clear indication that unpaired chromosomes have been present earlier. From a section
stained in gentian violet, x 2000.
That the expected types of spore are, indeed, produced is readily ascertained either
by counting chromosome numbers at the close of meiosis or by examining the early
mitoses in the germinating spores. The latter is the less laborious task, for observations
can be made within a week of sowing the spores. Two sample chromosome counts
showing 33 and 27 chromosomes respectively in spores from the triploid are given in
Fig. 2'ja and b.
Table 5 summarizes chromosome counts obtained in several successive years among
spore sowings from the triploid and the preponderance of grossly unbalanced types is
exactly as expected. Comparable figures for spores from the tetraploid are given in
Table 6 and again expectation is reaUzed. Gross unbalance is this time absent, but
a high proportion (approximately two-thirds) of the spores which begin to germinate
have one, or a few, chromosomes too few or too many. This proportion, it may be noted,
resembles very closely the relative proportion of mother cells containing trivalents
(Table 4).
39
THE POLYPLOID SERIES IN OSMUNDA
_ ■<-■/
^
c:-aMinp -W^^^w*-**
!8v.
Fig. 26. Comparison of germination in diploid, triploid and tetraploid Osmunda. x 40. a. Diploid
prothalli from the tetraploid. b. Prothalli derived from the triploid, almost all abnormal and
many non-viable, c. Normal haploid prothalli from the diploid.
Fig. 27. Chromosomes in descendants of triploid Osmunda. Permanent acetocarmine. x 500.
a, b. Mitosis in two different germinating spores showing 33 chromosomes [a) and 27 chromosomes
{b). c. Meiosis in a sporophyte produced from a spore sowing from the triploid showing presence
of one extra chromosome which makes a single trivalent among the otherwise normal pairs. For
further description see text.
The subsequent fate of these young prothalli is a matter of considerable interest.
The progeny from the tetraploid have not been closely followed, though it would
be expected that some unbalanced sporophytes must certainly be represented in the
next generation. The situation in the triploid has, however, been studied as closely as
circumstances would permit, because it might be expected that unbalance on the
scale shown might lead to the production of fundamentally new types of plant if a
prothallial culture of the type obtained could be self-fertihzed. That this expectation
was not fulfilled provides a highly instructive example of the power and mode of opera-
tion of natural selection.
The uneven appearance of a culture of spores from the triploid at the age when
chromosome counts can most easily be made is shown in Fig. 26^, and this uneven-
ness is never effaced. Many of the spores which start to grow proceed no further,
40
THE POLYPLOID SERIES IN OSMUNDA
Table 5. Chromosome numbers of germinating spores derived from autotriploid
Osmunda (3n = 66)
Chromosome number
22
23
24
25
26
27 28
29
30 31
32
33
34
35
36
37
38
•
•
•
I
I
I I
I
I
3 I
I
3
I
I
5
I
I
2
•
I
I
I
No. of spores
15 spores, 1942
10 spores, 1943
3 spores, 1944
Total 28 spores ... 1121141572. 11
Table 6. Chromosome numbers of germinating fresh spores of autotetraploid
Osmunda (4^ = 88)
Chromosome number
No. of spores
39
40
41
42
43
44
45
46
47
48
14 spores, 1943
10 spores, 1944
I
I
3
I
I
4
I
5
3
I
2
I
Total 24 spores
I
I
3
2
5
8
I
2
I
.
while others reach adult stature but are misshapen and imperfect, and though they
may live for years are unable to complete their life cycle. This is, indeed, the fate
of the majority, and offspring can only be obtained at all if spores are sown on
a very large scale.
As a result of repeated large-scale sowings and much patience, twenty-four descen-
dants of a triploid plant have been accumulated. They are very varied in appearance
as Fig. 28 will show; some died young and others seem unable to develop fertile
fronds although of an age to do so; a few are perfectly normal. The distribution
of chromosome numbers among them is highly significant. Had the average type
of spore reached sexual maturity it would be expected that the triploid chromosome
number would predominate in the progeny, although of course not the original triploid
constitution. This is, however, not the case. As summarized in Table 7, the majority of
such plants approximate to the diploid condition as regards chromosome number,
though the prevalence of structural abnormalities (Fig. 28) is sufficient evidence that
they are not all strict diploids of the normal kind.
Table 7. Chromosome numbers of young sporophytes raised by sexual reproduction from spores
derived from autotriploid Osmunda (3^ = 66)
Chromosome number
A
No. of plants c. diploid c. triploid c. tetraploid
24 21 2 1
The degree of approximation to diploidy has only been exactly determined in
those plants which had become fertile up till 1944 when the observations had to be
discontinued. These amounted to six plants, of which two were exactly diploid, one
41
THE POLYPLOID SERIES IN OSMUNDA
was a diploid plus i, i.e. with 45 chromosomes instead of 44 (see Fig. 27c), and two
were diploid plus 2, i.e. with 46 chromosomes. From the root-tip counts there is no
reason to think that any of the remaining diploids which had not yet become fertile
would show a greater degree of cytological unbalance
than this, and none was deficient in chromosomes.
It therefore seems certain that of all the prothalli
formed, only those deviating in not more than two
chromosomes from the haploid or diploid state were
able to complete their life history. In other words
natural selection, acting on the single character of
fitness to reproduce, had at one blow eliminated all
but a tiny minority of the prothalh brought under its
influence.
Natural selection is evidently acting in this case in
a capacity which is the exact opposite of a progressive
evolutionary influence; it is clearly a powerful force
tending to eliminate aberrations and to maintain the
stability of the species unchanged. Had the autopoly-
ploid series in Osmunda originated in nature, as it could
possibly have done, it therefore seems probable that it
would not appreciably have aflfected the evolution of
the species, for the tetraploid seems unhkely to be
able to compete with the diploid in the wild state,
and the triploid would be eliminated quite rapidly by
its sterility.
Disappointing as this conclusion may perhaps
appear, the facts described have a very considerable
comparative value which can at once be utilized.
Looking downwards to the Bryophyta it is clear that
the Osmunda series provides a very close parallel to
the situation induced in many of the mosses by the
Marchals, F. von Wettstein and others. In both groups polyploidy can be obtained at
will as an inevitable consequence of induced apospory, and in both the series is cut
short by sterility when a tetraploid gametophyte has been reached. Only in the
exceptional case of species hybrids, notably that between Funaria and Physcomitrium,
has an octoploid gametophyte been achieved, but to this there is as yet no parallel in
the ferns.
Looking upwards to the Flowering Plants it is clear that the cytological manifesta-
tions of polyploidy in Osmunda are of exactly the same type as in those Dicotyledons
and Monocotyledons in which they were first elucidated; multivalents are as well
displayed as in Datura, pachytene pairing closely resembles Lilium or Tulipa. There is
therefore direct evidence to justify the extension to the Pteridophyta of a type of inquiry
first founded on experience in the higher plants. This in itself is an encouraging
beginning.
Fig. 28. Leaves from two of the
progeny from triploid Osmunda
showing leaf aberrations. Both
plants were three years old at the
time the leaves were taken, but the
left-hand plant remained perma-
nently in the juvenile stage. For
further description see text.
Natural size.
42
THE POLYPLOID SERIES IN OSMUNDA
SUMMARY
A brief account of the salient facts in the origin, morphology, cytology and repro-
ductive effects of the autopolyploid series in Osmunda regalis has been given with
essential details illustrated photographically. The series consists of haploid, diploid,
triploid and tetraploid gametophytes and diploid, triploid and tetraploid fern plants,
together with a limited number of their descendants. The information so obtained is
intended in part as a standard of reference to aid in the interpretation of the facts to be
elucidated elsewhere in the Pteridophyta. The evolutionary importance of the auto-
polyploid series itself is believed to be nil.
43
CHAPTER 4
THE MALE FERN DRTOPTERIS FILIX-MAS
A very natural starting point to any inquiry involving observation of our native ferns in
the field is the Common Male Fern Dryopteris {=Lastrea = Nephrodium = Aspidium)
Filix-mas (L.) Schott. To quote a popular handbook (Step, 1908), it is said to be the
'commonest and best known British fern', and the inevitability of its recurrent use in
elementary classrooms may even at times have engendered that extreme measure of
superficial famiharity which breeds contempt.
Little might one suspect that beneath this apparently famihar surface lies a welter
of so much concealed complexity that a beginner in cytological matters might well
be engulfed like Christian in the Slough of Despond and be tempted to abandon the
whole project in despair. D. Filix-mas (L.) Schott is in fact not a species in any legitimate
sense of the word, except perhaps the 'coenospecies' of Turesson.* It is an assemblage
of forms differing in morphology, genetical constitution and life history, and there
seems little doubt that, among the aggregate of forms found wild in Great Britain,
at least three taxonomic species should be separately distinguished, and more may be
expected to be found in other parts of the world.
In order to unravel the position even in outline, cytological and other observations
have been made on about a hundred selected plants from the British Isles with a smaller
quantity of material from the continent of Europe. The scope of this inquiry had to be
somewhat restricted owing to the war, but enough of the necessary breeding work was
completed to reveal a position of unusual interest to the evolutionist, partly because
some of the larger problems raised have hope of solution.
The form of the species to which the name D. Filix-mas should still adhere when all
necessary subdivision has been carried out is a large plant, common in hedgerows,
woods and ditches throughout Great Britain, though probably not quite so abundant as
the almost ubiquitous species, D. dilatata, which will be discussed in the next chapter.
Any large population will show numerous small variations in form, an index no doubt
of slight internal genetical diversity, but in addition to the broad morphological
features fisted below, which all of them share, must be added a sexual reproduction of
the usual type (Fig. 42<:, e), and chromosome numbers of 164 for the sporophyte and
82 for the gametophyte (Fig. 30). Forbidding as such numbers may perhaps at first
sight appear, they are established with complete numerical accuracy and will be met
with repeatedly on later occasions.
The principal morphological characters by means of which D. Filix-mas in the
restricted sense can be distinguished in the field from the other species previously
amalgamated with it are fisted on the next two pages :
* A definition of this will be found on p. 71.
44
THE MALE FERN DRTOPTERIS FILIX-MAS
Fig. 29. Mature fertile pinnae of the three taxonomic species formerly included in the Male Fern
{Dryopteris Filix-mas sensAat.). Natural size. From left to right D. abbreviata (Lam. & DC.) Newm.
from the Mourne Mountains (east Ireland), D. Filix-mas sens. strict. emend., and triploid D. Barren
Newm. Further description in text.
(i) The leaf texture is softly herbaceous* as opposed to coriaceous,* and decay in
autumn is fairly rapid in consequence.
(2) When viewed from above the leaf is flat or convex but not concave (as m
D. abbreviata) .
* The words 'papery' and 'leathery' have been frequently used for these characters in the literature
for amateurs.
45
THE MALE FERN DRTOPTERIS FILIX-MAS
(3) The pinnules are completely separated, toothed and tapering (unlike D. Borreri).
Cf. Fig. 29.
(4) The margin of the indusium in the young state lies flat on the surface of the leaf
and is not tucked under the sorus.
(5) The sori are fairly large, the average diameter being 1-5 mm. (contrast with
D. abbreviata).
(6) The ramenta on the rachis are sparse.
^* ^"^ «r*
D. Filix-mas n=82
Fig. 30. Diagram to explain Fig. 35. x 1500.
In contrast with this hedgerow type, in which it is not at the moment profitable to
distinguish varieties, though with fuller genetical knowledge this could perhaps be
done, it is important to separate a smaller mountain form with half the chromosome
number (i.e. 2^ = 82, ^ = 41, cf. Fig. 31), which is, nevertheless, also sexually repro-
duced (Fig. 42^) and can be made to hybridize with the other (see p. 49 below). In
many Floras it goes by the name of 'var. abbreviata Newman', and has been recorded
from most of the mountainous regions of England, Scotland and Wales, and from two
localities in Ireland ; it is also said to have been found in central France.
The principal characters by which D. abbreviata * can be distinguished in the field are
as follows :
( 1 ) The habitat is that of a mountain plant with a marked preference for the shallow
soil of rock crevices or scree; it is only to be found under trees where these have invaded
a previously exposed site.
(2) Its stature is smaller than that of D. Filix-mas and the form of the stock is much
more tufted owing to frequent branching. A stock with a single crown is only usual
among young plants.
* Some other details are enumerated by Wollaston (1875).
46
THE MALE FERN DRTOPTERIS FILIX-MAS
(3) The leaves are stiffer in texture than those of D. Filix-mas, though they become
more hke that species under shade conditions. Under normal conditions of exposure
their relatively slow rate of decay results in an unusually conspicuous hanging mass of
russet-coloured dead leaves, visible below the functional crown at all seasons of the
year.
(4) One of the best characters to observe in |^^ ft^kA
the field, though, unfortunately, one which is Ofc^A
lost in a herbarium specimen, is the concavity of ^^ ^m ^ ^^^
the frond when viewed from above. The tips and 0 ^ m
edges of the leaves curl upwards conspicuously wri ^
in a young frond and never quite flatten out in ^ ^ ^
an old one (cf. Fig. 29). In all other forms of the |^ % V A
Filix-mas complex the pinnae are either flat or A ^ ^ ^M
ve recurved edges. A* ^C^ •^
(5) Average size of the sori does not as a rule ^ ^ *"^y
have recurved edges. A* ^C^ ^b
exceed i mm. and is therefore smaller than in D abbrevi^^a ^ ^
D. Filix-mas. Oddly enough the spores are almost "' ^. , • 17- c
' ^ ^ . . rig. 31. Diagram to explain rig. 30.
identical in size with those of D. Filix-mas ^ j^^^
(cf. Figs. 39-40)-
(6) There are numerous pale scales (ramenta) on the rachis.
Though the existence of D. abbreviata as a distinct form has been known for over
a hundred years, there has been much controversy about its status. This illustrates
so well the way in which a question of taxonomy may be insoluble without cytogenetic
information that it may be instructive to quote the literature in some detail as an
example of a type of situation which will meet us repeatedly in later pages.
In 1 81 5, Lamarck and de CandoUe described in their Flore Frangaise a small fern from
south-west France as ' Polystichum abbreviatum' in the foUowing words:
'On pourrait, au premier coup-d'oeil, prendre cette espece pour une simple variete
de la fougere male, mais elle est de moitie au moins plus petite; ses pinnules sont plus
courtes, plus obtuses, et presque d'egale largeur dans toutes leurs etendues : leurs lobes
sont plus larges, plus courts et moins nombreux, et chacun d'eux ne porte ordinairement
a sa base qu'un seul groupe de fructifications, tandis qu'on en trouve plusieurs a la
base de chaque lobe dans la fougere rnale.
'Cette plante a ete trouvee dans les Landes, par les C. Dufour et Thore.' {Fl. Fr. 11,
560.)
In Great Britain it is generally considered to have been Newman who first equated
de Candolle's 'species' with a British plant from Ingleborough in Yorkshire (1844,
History of British Ferns, 2nd ed., p. 202), while expressing doubts regarding its specific
distinctness. Moore, in 1848 [Handbook of British Ferns, ist ed., p. 43), regarded the
Ingleborough plant as definitely a variety of Filix-mas and called it accordingly Lastrea
Filix-mas var. abbreviata. Newman accepted varietal status for it in the third edition of
his History (1854) and called it 'de Candolle's Male Fern, Dryopteris Filix-mas var.
abbreviata'. In 1855, however, G. B. WoUaston, whose knowledge of British ferns, in
Newman's words, 'infinitely exceeds that of any other botanist with whom I have ever
47
THE MALE FERN DRTOPTERIS FILIX-MAS
enjoyed the opportunity of conversing', again advocated very strongly that D. Filix-
mas and D. abbreviata should be regarded as distinct species and proposed the name of
Lastrea propinqua for the latter.
It is perhaps unnecessary to add further details, since the position to-day is sub-
stantially the same as that reached in 1855. There is slight doubt whether the British
form is really the exact equivalent of Lamarck and de Candolle's French plant, since
their description of a single sorus only, at the base of each pinnule, certainly does not
apply in Britain, but 'var. abbreviata Newman' is a commonplace of British as of
continental floras, and Wollaston's preference for specific distinctness has either not
been known or has been generally disregarded by botanists. Among amateur fern
collectors it is otherwise. Wollaston's name ''propinqua^ was accepted at once and is
still in common use by members of the British Pteridological Society, and it figures in
many collector's handbooks, including one as recent as that by Druery (191 2). By the
modern International Rules of Nomenclature* the name itself is illegitimate, since,
among other things, it had been utilized for other plants on at least two previous
occasions, namely, in 1841 by J. Smith and in 1849 by Presl. Wollaston's view of
specific distinctness has, however, been fully borne out by the cytogenetical facts to be
recorded below, and the only necessary modification is to admit the prior claim of the
name abbreviata and to designate the species, if it is to be a species, as Dryopteris abbreviata
(Lam. & DC.) Newman.
The cytogenetic confirmation of the correctness of the separation of D. abbreviata
from D. Filix-mas is based on several lines of evidence. In the first place the difference
of chromosome number, one being half the other, is very constantly displayed (cf. Figs,
34^, 36). The haploid complement of 41 chromosomes has been found in plants bearing
the morphology of Z). abbreviata from the following localities:
Wales: near Bala and near Bettwys-y-Coed ; Lake District: Kentmere Valley and
Borrowdale; Scotland: Greenhill Dod near Glasgow; Ireland: Mourne Mountains
(east coast) and Brandon Mountain (west coast).
An even stronger argument for the specific distinctions of D. abbreviata is that when
crossed with D. Filix-mas it forms a highly sterile hybrid.
The setting up of species hybrids is not an easy matter in ferns and it is always
slow. The presence of both sexes on the same prothallus makes the risk of accidental
self-fertilization greater than is usual, though it can be minimized by using only old
prothalli which have been watered from below, to serve as females. The success
or otherwise of the cross cannot easily be determined for some months after an insemina-
tion has been made owing to the morphological similarity of all related young ferns in
the early stages. If a hybrid can be detected as such by its possession of a chromosome
number different from that of the female parent, it can be detected by a root-tip
count in about a year from its inception. Meiosis cannot be hoped for till the plant is
ab out 3 years old.
* English-speaking botanists who are not professional taxonomists will find a very helpful introduc-
tion to the International Rules of Nomenclature in Bisby (1945), in which the Rules and Recommenda-
tions including those added at the 1935 International Congress at Amsterdam are reproduced verbatim.
I am much indebted to F. Ballard of Kew for drawing my attention to this very useful little book.
48
THE MALE FERN DRTOPTERIS FILIX-MAS
Hybrids between D. Filix-mas in the restricted sense and D. abbreviata were success-
fully produced in 1939 using spermatozoids of a D. abbreviata from Greenhill Dod near
Glasgow (Fig. 32 c) and archegonia of a D. Filix-mas irora Ingleborough, Yorkshire
(Fig. 320). Two hybrids were obtained out of six inseminations, the other four remain-
ing without offspring. The hybrids were attested as such by showing a chromosome
number intermediate between that of the two parent sporophytes {2n = c. 120), and
Fig. 32. Comparable pinnae, natural size, of the triploid hybrid {b) between Dryopteris Filix-mas
sens. strict. emend, from Ingleborough {a) and D. abbreviata (Lam. & DC.) Newm. from Greenhill
Dod near Glasgow (c).
meiosis has since been seen in both. The first sporangia were produced on one of the
plants in 1943, though, unfortunately, it died in the following winter; the other was
first fertile in 1944, and had become a large plant by 1948 when the silhouettes of
Figs. 326 and 33 were taken. In pressed condition it is so like D. Filix-mas that it
would probably be mistaken for that species but for its abortive spores and abundant
ramenta. It is, however, definitely intermediate between its two parents in most charac-
ters as Fig. 32 may perhaps suggest. The frond is slightly concave when viewed from
above, though less so than is D. abbreviata. The stock has, however, become much
MFC
49
Fig. 33. The triploid hybrid between Dryopteris Filix-mas sens. strict. emend, and D. abbreviata (Lam. &
DC.) Newm.; a larger portion of the frond from which the pinna of Fig. 32 b is taken, to show general
morphology and ramenta on the rachis. Natural size.
THE MALE FERN DRYOPTERIS FILIX-MAS
branched as in that species. No viable spores have so far been obtained from it, though
sowings have been made several times.
It may be mentioned in passing at this point that almost all attempts at defining the
species concept include recognition of the necessity of some measure of morphological
d e
Fig. 34. Samples of meiosis in the Male Fern complex from sections stained in haematoxylin. x 1000
a. Dryopteris abbreviata Newm. Compare with Fig. 36 for details, b. Triploid hybrid between
D. abbreviata and D. Filix-mas showing lagging unpaired chromosomes, but also numerous pairs.
Compare with Fig. 37 for details, c. D. Filix-mas sens. strict. emend. Compare with Fig. 35 for
details, d. Diploid D. Borreri Newm. Note the large size of the spore mother cells which compares
with that of D. Filix-mas rather than with D. abbreviata. For explanation of this see p. 58.
e. Pentaploid D. Borreri x D. Filix-mas, a wild hybrid. Note much larger cells than in d and very
densely crowded metaphase plate.
distinctness in the species and, usually, of some element of sterility in crosses with
other, even though related, species. On the evidence already presented therefore
D. abbreviata is almost certainly a species.
Additional information is obtained by study of meiosis in this hybrid. As already
made clear in previous chapters, the numerical details of chromosome pairing at
51 . 4-2
THE MALE FERN DRYOPTERIS FILIX-MAS
meiosis can be a very valuable guide to the genetical make-up of a plant. In this
way the only condition under which D. abbreviata and D. Filix-mas could not legiti-
Fig. 35. Acetocarmine squash of Dryopteris Filix-mas sens. strict,
emend, showing 82 pairs of chromosomes. For explanatory
diagram see Fig. 30. x 1000.
s
4
*7^
Fig. 36. Acetocarmine squash of
Dryopteris abbreviata Newm.
showing 41 pairs of chromo-
somes. For explanatory dia-
gram see Fig. 31. x 1000.
•
A*
'*<
'^
<n\*
**^'
% t
1»
S%
* #*
-••v
>C
• *
%
%
e
*t*
Fig. 37. Triploid hybrid between Dryopteris Filix-mas and D. abbreviata. Acetocarmine squash showing
40 univalents, 40 pairs and a trivalent. x 1500. For explanatory diagram see Fig. 38.
mately be separated taxonomically would be if it could be shown that one was
merely an autotetraploid form of the other. In that case, however, one would expect
to have found quadrivalent groups in B. Filix-mas, but these are certainly absent.
52
THE MALE FERN DRTOPTERIS FILIX-MAS
One would also expect numerous trivalents in the triploid hybrid, which, however,
also do not occur.
Meiosis in the triploid hybrid between D. Filix-mas and D. abbreviata is shown in
Fig. 34^, in which some cells at the first meiotic division are put between comparable
views of the two parent species. The irregularity produced by lagging unpaired
chromosomes is very conspicuous in the hybrid. The details of chromosome pairing are
better displayed in a 'squash' preparation, and comparable cells of the three plants
are given again in this technique in Figs. 35-37. Fig. 37 is reproduced at a higher
magnification than Figs. 35 and 36 to facihtate observation, and a diagram of the
CD o ^
^ .<%
0
0
o
¥
0 V>^V .^0
^
^ ^ 0 ^^oo
o
Triploid hybrid 3n -- 123 ^ ^U ^
Fig. 38. Explanatory diagram to Fig. 37. x 2000. Pairs in black, univalents in outline.
complete analysis of it is given in Fig. 38. The 123 chromosomes contained in it
are represented by 40 pairs, 40 univalents and i trivalent. This is not the type of
pairing found in autotriploid Osmunda but is closely comparable to that of allotriploid
watercress, allowance being made for the different monoploid number {n = ^i in
Dryopteris). The single trivalent is most easily explained by postulating one segmental
interchange between two otherwise non-homologous chromosomes in the D. Filix-mas
nucleus. Except for this the pairing bears a very close numerical relation to the basic
haploid number, and the interpretation would appear to be, as in the case of the
watercress, that all the chromosomes of the diploid species can find partners in the
tetraploid species but that these represent only half the nucleus of the latter, the other
half apparently appertaining to some different species with different homologies in its
chromosomes.
53
THE MALE FERN DRYOPTERIS FILIX-MAS
The 'commonest and best known British fern' would therefore seem to be the first
case of an allopolyploid to be detected among ferns and to have owed its origin in the
first place to a cross between a species identical with or closely resembling D. abbreviata
and some other diploid species which has not yet been identified, followed in the
second place by chromosome doubling in the hybrid.
If this diagnosis is correct it should be possible at some future time to resynthesize
the Male Fern from its component species. The identification of the second parent is,
however, hkely to be a matter of some difficulty. D. Filix-mas in the narrow sense is
common all over Europe, extending east at least as far as the Caucasus. It must there-
fore have been in existence for some time, and the second parent may in fact be extinct.
At least it may be expected to be non-British, since otherwise it seems likely to have
been detected in some way by the sharp eyes of the early collectors. This problem may
therefore be recommended to the interest of continental botanists.
Fig. 39. Spores oi Dryopteris Filix-mas sens. strict. emend, x 100.
The third species that should be separated from the old Filix-mas complex cannot
be fully dealt with until the cytological details accompanying apogamy have been
described (see Chapters 10 and 11). It is D. Borreri Newman, and its principal diagnostic
characters known to me in Great Britain are as follows : *
( 1 ) Usually a large and handsome fern (except for certain local strains) and in both
respects more striking than D. Filix-mas.
(2) Fronds tough and 'leathery', persisting long in the autumn and sometimes
surviving the winter, often of a more yellowish green than in the other two species.
The vernation of the unfolding fronds is usually more lax than in the other species.
(3) Surface of the pinnae glossy when living.
(4) Ramenta very abundant and more so than in either of the other two species.
Often, though by no means always, of a bright orange yellow, sometimes with some
darker cells at the base of the scale.
(5) Indusium in most strains tucking under the sorus.
(6) Shape of the pinnules very characteristic ; they are not completely separated at
the base; the sides of the pinnules are rather straight and the tips abruptly truncate
and not tapering (cf Fig. 29).
(7) A very characteristic detail always present, even in putative hybrids, though
unfortunately lost in herbarium specimens, is the presence of a patch of dark pigment on
* For further amplification, see Wollaston (1875).
54
THE MALE FERN DRTOPTERIS FILIX-MAS
the base of the pinna rachises near where these join the main rachis. This character
was first pointed out by Newman, and I have also found it very rehable even in hybrids
with D. Filix-mas.
(8) The spores are consistently larger than in either of the other species, though some-
times much admixed with abortives for reasons which will appear. Figures will be
found in Chapter 1 1. As a rule only 8 spore mother cells instead of i6 are produced in
one sporangium.
(9) Prothalli consistently apogamous (cf. Fig. 42 a, d).
(10) Habitat closely resembling D. Filix-mas, though with local strains accompanying
Fig. 40. Spores o{ Dryopteris abbreviata Newm. x 100.
Fig.
4 1 . Spores of diploid Dryopteris
Borreri Newm. x 100.
D. abbreviata. Common all over Great Britain, often forming almost pure stands covering
large areas. In these cases the appearance of a local population is very uniform, and the
feeling of individual differences so easily detected in D. Filix-mas is quite absent. Popula-
tions characteristic of different localities may show differences of detail (see below).
The history of Z). Borreri, like that of Z). abbreviata, is a record of a centiu^y or more of
dispute. Many synonyms were already pointed out by Newman in 1854, the plant
having been generally recognized both in Great Britain and on the Continent as a
very distinct 'variety' (cf Babington's Manual of British Botany). Its claim to specific
rank has been repeatedly made, notably by Wollaston, who proposed the name ofLastrea
pseudo-mas for it in 1855. This name is illegitimate, but as an index of the amount of
attention that the idea of the species has received from continental botanists (unlike
Dryopteris abbreviata) it may not be without interest to append a translation of a short
note which appeared in the Proceedings of the Swiss Natural History Society, published in
Geneva in 1937 {Verh. schweiz- naturf Ges. 1937, p. 153):
'THE RANGE OF FORM IN DRTOPTERIS BORRERI NEY^M.'
By Franz von Tavel (Bern)
'The author, in collaboration with E. Oberholzer of Ziirich, has attempted to sum-
marize the wealth of forms within Dryopteris Borreri, a group of ferns which is generally
included in floras under the now obsolete names of Z). Filix-mas var. paleacea (Moore)
Druce, and var. subintegra (Doll) Briquet. The results are as follows:
I. D. Borreri Newm. s.str. Indusium stiff, leathery, turned inwards at the edge,
sometimes splitting in a few sori.
55
THE MALE FERN DRTOPTERIS FILIX-MAS
(i) var. atlantica v. Tavel n.var. Sori small, up to i mm. wide, frond stiff and very
tough — Madeira and Spain. Near this is f. Merinoi Christ {Bull. Acad. Int. Geogr. hot. Le
Mans, no. 172, 1904) in Spanish Galicia.
(2) var. Duriaei Milde {Fil. Europ. et Atlant. 1867, 123) in Asturias.
(3) var. insubrica v. Tavel n.var. with large reddish brown indusia, touching each
other, fronds a normal green, somewhat hairy — Tessin, south side of the Simplon;
Unterwallis near Salvan (Coquoz, Farquet). In addition, conspicuous in the Bergamo
Alps (Chenevard), Liguria [Erb. Crittog. Ital. 605) and Corsica (Aellen). With smaller
sori also in the Black Forest; Baden-Baden (M. Lange) and Zastlertal (Losch).
(4) var. disjuncta Fomin (Moniteur, Jard. Bot. Tiflis, xx, 27, 1911). The most highly
developed form which has split indusia the most frequently — Tessin; Upper Rhone
(Oberholzer) ; Black Forest (Christ, Losch); Vosges (Walter); Caucasus (Fomin). f.
paleaceolobata (Moore) {Oct. Nat. -print. Ferns, i, 195, pi. 33 C). Segments of the lower
pinnae incised — Tessin; Upper Rhone (Oberholzer); England; Channel Islands;
Scotland,
(5) var. pumila (Moore) {Ferns of Great Britain and Ireland, pi. 17 B, 1855). Alpine
dwarf form with glandular indusium. Sori in a single row — Wales. South side of the
Simplon ; Tessin, in part in transition forms to var. insubrica.
(6) var. rubiginosa Fomin, loc. cit. 29. Not known with certainty beyond the
Caucasus.
(7) var. melanothrix v. Tavel n.var. Leaf texture soft, with a dense indumentum
composed of long patent black filamentous scales mixed with colourless lanceolate
ones. Indusia small, black. Dillingen in the Saar (W. Freiburg).
II. Related forms with the habit of the first group, but with the deciduous fiat
indusium found in Dryopteris Filix-mas.
(8) var. ursina (W. Zimmermann) {Allg. Bot. ^. 22, 1916). Parallel form to var.
insubrica, differing in the indusium. This is the form which is generally referred to as
var. subintegra, but this name also includes var. disjuncta and other forms when used by
Doll and Christ. — Widely distributed in the Alps in woods up to 1700 m. (Davos)
also in the Black Forest and the Vosges. Near to this is f. aurea v. Tavel f.n. apparently
a subalpine handsome form of yellow-green colour — Upper Rhone (Oberholzer) ;
Bernese Oberland; Pont de Nant (Wirtgen).
(9) var. pseudodisjuncta V. Tavel n.var. Habit of var. disjuncta, indusia ofZ). Filix-mas
— central Switzerland (Oberholzer) ; Bernese Oberland.
(10) var. tenuis v. Tavel n.nom. (syn. var. subintegra Fomin, loc. cit. 29. Aspidium
Filix-mas var. subintegrum Doll. p.p. Christ p.p.)- Spores in Swiss material partly aborted
(Oberholzer). — Upper Rhone (Oberholzer); neighbourhood of Bern; Schaffhausen
(Kummer) ; neighbourhood of St Gallen.
(11) var. robusta v. Tavel n.var. In various ways intermediate between D. Borreri
and the different varieties of Z). Filix-mas, with the leaf texture and hairs of the first and
the pinnule shape and indusium of the second. Possibly hybrids between the two but
fertile. — With the two parent species in mountain woods. Upper Rhone (Oberholzer) ;
Bernese Oberland; Unterwallis (Coquoz); Black Forest, Hirschsprung (Losch).'
One of the principal reasons for the very large number of apparently true-breeding
varieties and forms within D. Borreri undoubtedly is the persistent apogamy of the
species. Any morphological variant produced by mutation, hybridization or other
56
THE MALE FERN DRYOPTERIS FILIX-MAS
means, can persist and multiply if not adversely affected by selection. Local popula-
tions may therefore virtually be single clones, and their appearance of extreme
uniformity already commented upon is almost certainly an expression of this.
Fig. 42. Sexual and apogamous prothalli. a. Median longitudinal section through a prothallus of
diploid Dryopteris Borreri showing apical cell of prothallus and an early stage of an apogamous
embryo behind, x 100. b. The same through a sexual prothallus of Z). abbreviata Newm. showing
central cushion bearing archegonia and glandular hairs on the lower surface, x 100. c. The
same through a sexual prothallus of D. Filix-mas sens-strict. emend, showing glandular hairs and
young archegonia on the central cushion near the apex, x 100. d. Whole mount in glycerine
jelly of a prothallus of diploid D. Borreri Newm. at the stage sectioned in a. The apogamous embryo
appears like a dark spot, x 10. e. The same of a sexual prothallus of Z). Filix-mas with archegonia.
The principal characteristics by which apogamy in these ferns can be diagnosed are
as follows :
(i) The chromosome number of the prothalli is the same as that of the parent plant,
no matter what this may be.
57
THE MALE FERN DRTOPTERIS FILIX-MAS
(2) Sex organs are imperfect or absent, the antheridia being usually functional but
the archegonia absent.
(3) The central cushion of the prothallus remains thin, and at a very early age
(compared with the time needed for a sexual prothallus) the various organs of a new
sporophyte are produced directly from it (cf. Fig. 42 a and d).
(4) The first leaf of an apogamously produced plant resembles the second or third
leaf of a sexual one, the very simple first leaf of the normal sporophyte being un-
represented (cf. Fig. 43 ^).*
a b
Fig. 43-
Fig. 44.
Fig. 43. Prothalli bearing young plants each with a leaf and a root expanded, a. Dryopteris Filix-mas
produced sexually, b. Triploid D. Borreri produced apogamously. Note the difference in the form
of the first leaf. Twice natural size.
Fig. 44. A pinna of Dryopteris Borreri var. polydactyla Wills. Natural size. Formerly known as
D. 'pseudo-mas' var. polydactyla Wills, a classic horticultural monstrosity originally found wild as
a single specimen but preserved in cultivation on account of the curious forked apices. A very
similar specimen known in horticulture as var. polydactyla Dadds is illustrated in Fig. 195, p. 187;
var. Wills is diploid (see below), var. Dadds is triploid (see Chapter 11).
(5) The good sporangia generally liberate only 32 large spores instead of the 64 of
normal ferns. In addition to 'good' sporangia, others with aborted spores in various
numbers can generally be found (see Chapter 8 for further details) .
(6) In the ripening of the sporangia there are some characteristic aberrations affecting
the last (premeiotic) mitosis (for further details see Chapter 10).
Apogamy, detected by one or all of these various criteria, has been found in every
plant examined bearing any of the D. Borreri morphological characteristics from over
twenty different locahties in the British Isles. In addition, I was fortunate in receiving
* This character was first noticed by Stange (1887) at a very early stage in the history of apogamy.
58
THE MALE FERN DRTOPTERIS FILIX-MAS
from Dr von. Tavel in 1939 spore samples of seven of his Swiss varieties and forms.
These were var. insubrica, var. disjuncta, var. ursina, var. ursina f aurea, var. tenuis, 'var.
tenuis nigricans', var. robusta, 'var. punctata'. AU of these were apogamous, and apogamy
had previously been recorded by Dopp (1939) in some plants from Germany.
It would be a matter of considerable interest to know if sexual forms of this species
exist, but from their obvious scarcity the suspicion is aroused that they do not. If this
were known with certainty to be the case, a very interesting problem would be raised
as to the possible origin of these plants. Apart from undoubted hybrids with Filix-mas
(and perhaps with D. abbreviata), which will be referred to below (see also Chapter 11),
the morphological characteristics of D. Borreri concern a number of organs and both
morphological generations. A simple mutant origin of all these characters at once seems
somewhat improbable. An alternative explanation might be that both apogamy and
morphological characters were produced simultaneously by an act of hybridization,
the incidence of apogamy making the hybrid stable from the outset without other
cytological change (for further discussion of this, see Chapter 11).
Be that as it may, it is certain that some cytological changes have taken place
subsequently. Within the D. Borreri complex a whole range of chromosome numbers,
in polyploid series, can be found. In the Swiss material var, disjuncta and 'var. punctata'
were diploid and the remainder triploid. In Great Britain the majority of plants are
triploid (Figs. 46, 48) but extensive local diploid populations have been met with in
Ireland, Wales and the Lake District. Diploids and triploids are so much alike that
I have not succeeded in detecting any constant morphological differences that will serve
for their identification in the field in a new locality, except perhaps spore size which is
greater in the triploid than in the diploid, and it is significant that when pressed fronds
of British plants were submitted to Dr von Tavel, a diploid from north Wales was
identified as var. insubrica which in Switzerland was triploid, with only the remark that
the sori in the Welsh specimen were smaller.
Without at the moment discussing the possible origin of polyploidy within D. Borreri
which will more easily be dealt with at a later stage (cf Chapter 11), an extension of
the polyploid series by subsequent hybridization with D. Filix-mas can nevertheless
be detected. An artificially produced hybrid between D. Filix-mas as female parent
and a horticultural strain of diploid D. Borreri with forked leaves, was described by Dopp
in 1939, the hybrid being identified as such with complete certainty by having the
forked fronds and apogamous reproduction derived from its male parent, but combined
with the tetraploid and not the diploid chromosome number. The output of 'good'
spores was also much reduced. I have not myself repeated Dopp's experiment for the
reasons explained in the Preface, but it is not unusual to find individual specimens of
closely comparable type where populations of the two species are growing together in
the wild state. Such plants will show the wealth of ramenta and the dark spot on the
pinna rachises characteristic of D. Borreri combined with the pinna shape and some
other characteristics of Z). Filix-mas. The spore output is low, but such 'good' spores
as may be formed, which are always very large, give rise to apogamously reproducing
prothalli. In two cases found by myself the chromosome number of such plants
proved to be tetraploid, as in Dopp's hybrid, but two other cases picked out on their
59
r^
>
-ii
Fig. 45. Dryopteris Borreri var. polydactyla
Wills, a diploid form of D, Borreri
with 82 chromosomes, from an aceto-
carmine squash, x 1000. For ex-
planatory diagram see Fig. 47.
Fig. 46. Triploid Dryopteris Borreri, normal form.
Acetocarmine squash, x 1000. For explanatory
diagram see Fig. 48.
3<
^% QjCjf
B. Borreri van polijdacft/la Wil/s "n "= 82
Fig. 47. Explanatory diagram to Fig. 45.
X 1500.
D. Borreri "n "= 123
Fig. 48. Explanatory diagram to Fig. 46. x 1500.
THE MALE FERN DRYOPTERIS FILIX-MAS
morphological characters by Dr L. Praeger were pentaploids. A meiotic metaphase plate
of one of the latter is illustrated in Fig. 34^ with a comparable figure of a diploid
D. Borreri (Fig. 34^) added for comparison. Some photographs of the spores will be
found in Fig. 201, p. 192, Chapter 11. Reduced fertility, mixed morphology and
chromosome number all support a hybrid origin for these plants, the most probable
parentage for the tetraploids being diploid D. Borreri (male) xZ). Filix-mas (female),
while that of the pentaploids seems necessarily to be triploid D. Borreri (male) x D. Filix-
mas (female) . The fact that both types of hybrid can to a limited extent breed true
by means of the persistence of a few apogamously reproducing spores makes them
somewhat more conspicuous elements in our flora than is usually the case with newly
Fig. 49. Map of the northern Hmits of Dryopteris Borreri Newm. in Europe.
After Nordhagen (1947).
formed species hybrids, and their existence is no doubt the chief reason why the specific
distinction of Z). Borreri has for so long been opposed by some taxonomists.
Summarizing these conclusions, without going further at present into the mariy
interesting problems raised by D. Borreri Newman, it is sufficient for the moment to
have shown that on the simple criteria of morphological distinctions and genetical
discontinuity, three taxonomic species should be distinguished in the Male Fern popula-
tions of Great Britain. Two of these species show polyploidy and both of these can be
proved, or suspected, to have arisen as a result of hybridity. All three species can
probably cross with each other, though the subsequent fertility of such crosses is low.
Confusing as all this may perhaps appear to a beginner and especially to an amateur
field collector, the choice of the Male Fern has turned out to be an unexpectedly
fortunate one for the purpose of the present inquiry. The use of the methods of
61
THE MALE FERN DRYOPTERIS FILIX-MAS
cytogenetics has been fully justified, and while it may be left to the reader to decide
whether or not 'the commonest and best known British fern' was really as simple as it
seemed, an answer in no uncertain terms has been received to our first general question.
By the time that we have diagnosed the existence of polyploid hybrids, of hybrids with
hybrids, it has become almost impertinent to ask for further evidence that polyploidy
and hybridity do at least exist in ferns.
SUMMARY
A brief morphological and cytological description is given of the three taxonomic
species into which the Male Fern complex should be split. One of these species,
D. abbreviata (Lam. & DC.) Newman, is a sexual diploid with a gametic chromosome
number of 41 ; it has various distinctive morphological characters, including small size and
an ecological preference for rocky localities in mountains. D. Filix-mas s.str. is a sexual
type with twice this chromosome number (i.e. a sporophytic number of 164 and a gametic
number of 82). It can be hybridized with D. abbreviata, and from the pairing behaviour
in such a hybrid it is deduced that the D. Filix-mas is itself an allopolyploid with half its
nucleus homologous with that of D. abbreviata and the other half of unknown origin.
D. Borreri Newman owes its specific distinctness to its very characteristic morphology,
in which it differs from both the others. It appears to be exclusively apogamous and
diploid, triploid, tetraploid and pentaploid strains are known, the last two being almost
certainly hybrids between the first two and D. Filix-mas. Further consideration of
D. Borreri will be found in Chapters 10 and 1 1.
62
CHAPTER 5
THE GENUS DRTOPTERIS* IN BRITAIN
Proceeding from the single species, or, as it has turned out species complex, to the
wider background of the genus, the other British members of Dryopteris may next claim
our attention. But here the diversity of forms is such, even in the limited flora of one
small island, that a simple narrative will at times have to give place to something
resembling a catalogue in which the logical coherence is supplied only by the fact that
each of the species listed has, until quite recently, been regarded as sufficiently akin to
all the others to be classified with them.
At first sight the dozen or so British species of Dryopteris (sensu lato) fall into two rather
distinct groups. On the one hand there are about eight species of the 'Buckler Fern'
type with kidney-shaped indusium, formerly placed in the genus Lastrea (or sometimes
Mephrodium or Aspidium). On the other hand, there are the three species with naked
sori and small creeping rhizomes popularly known as the Oak Fern, the Beech Fern,
and the Limestone Polypody. These are very different in habit from the British species
of ' Lastrea ' and were all formerly placed in the composite genus Poly podium owing to the
absence of an indusium to their sori. The inadequacy of this negative feature as the
basis of a genus has, however, been generally recognized for some time, and though
it is not alv/ays easy to determine the affinities of non-indusiate species with certainty,
it was Bower's (1935) opinion that the nearest indusiate relatives to the three species in
question were in Dryopteris.
That this simple description of the genus is an incomplete picture has, however, been
shown with surprising clarity by the cytological results. And since it will be necessary to
call attention on several occasions to the need for taxonomic revision, not of species
only but also of the genus, it may be well in the first instance to disregard all previous
impressions and deal with each species categorically in the order that its chromosomes
suggest.
Dryopteris aemula (Ait.) O. Kuntze {Nephrodium foenisecii Lowe) makes an appropriate
starting point. This very characteristic and beautiful species (Fig. 50) has a markedly
Atlantic distribution, being far more abundant in Ireland than in England, where it
is commonest in the west and south-west. I have investigated plants from Cornwall
and from Ireland with identical results.
Meiosis in Dryopteris aemula as seen in a section is reproduced in Fig. 7 1 b, but the
details of the chromosome count are better displayed in a 'squash' preparation as may
be seen in Figs. 51 and 53a. The chromosome number in both is exactly n = /\.i. This
* The classification adopted at the beginning of this chapter is that of Druce's Comital Flora (1932),
which follows Christensen's Index Filicum. Recommendations on generic subdivisions to suit the cytology
will be found at the end, and it is perhaps only fair to state that some of these have already been
introduced on other grounds by systematists (cf. Clapham's List 1946).
63
Dryopteris aemula (Ait.) O. Kuntze
from County Wicklow. Ireland.
I natura
size.
64
♦<
THE GENUS DRTOPTERIS IN BRITAIN
is of importance first in showing that D. aemula fits well into the cytological scheme of
the genus Dryopteris outlined already by the Male Fern story. The exact identity of
chromosome number between D. aemula and the diploid species D. abbreviata is, how-
ever, also of interest in showing that this somewhat unusual prime number must be
far older than the Male Fern complex itself, and it will be found, indeed, to characterize
not only these species but also several other related genera as the next chapter will
show.
A second somewhat isolated species is D. Villarsii (Bell.)
Woynar [D. rigida (Hoffm.) Underw.). This plant J^^
inhabits the deep cracks in the limestone pavement of SM
parts of the northern Pennines, but elsewhere in Britain ^^
it seems to have been recorded only from one doubtful ^J|P
station in north Wales. In Switzerland it occurs as an
alpine, and therefore its British station may be suspected O
to be that of a glacial relict. In Great Britain D. Villarsii ^ A
is a tetraploid species having w = 82 (Fig. 53^). As in ^ jj^
D. Filix-mas quadrivalents are absent. A somewhat un- ^^SJ ^^
expected observation made as a result of a personal ^^^M
visit to Switzerland in the summer of 1947 is, however, ^Ci^ ^KJ^
that the species in that country is diploid. Fixations ^^iJ^ ^
were taken both in the alpine garden of Pont-de-Nant sur • ^
Bex and on the wild plants in their natural habitat some 90
kilometres away. The number is unquestionably half
that of British material. This observation was obtained W^
too late to be given more than a passing mention before D.sejnula n = 4i
this manuscript is completed for the press, but it raises Fig. 51. Explanatory diagram
some very interesting problems concerning the past to Fig. 53a. x 1500.
history and nature of D. Villarsii in Britain, which will
perhaps be further pursued elsewhere.
The next group of species can best be treated together. They are the species complex
once united under the now obsolete name of ' Nephr odium spinulosum\'' but long since
resolved, by the common consent of taxonomists, into three good species and the
hybrids between them. The three good species are D. cristata (L.) A. Gray, D. spinulosa
(Miill.) Watt and D. dilatata (Hoffm.) A. Gray. They differ from each other in morpho-
logy, habitat and frequency of occurrence. D. cristata, the rarest of the three, is a
plant of very wet bogs, so rare as to be in danger of extinction in this country, no doubt
in part owing to drainage. It was formerly recorded from a few isolated stations in a
number of counties scattered from Dorset to the Glasgow area, but it appears to have
died out in most of these except Norfolk. It has, however, recently been discovered in
Surrey (Payne, 1939) and is still to be found near Glasgow. D. spinulosa, in contrast,
is found in most parts of the British Isles and is often abundant, though it is limited
* Ignorance of the biology and taxonomy of this group has led some recent writers on evolution,
notably Huxley (1942), to some very erroneous conclusions.
MFC 65 5
Fig. 52. Dryopteris Villarsii (Bell.) Woynar
(£). rigida (HofFm.) Underw.) from the
northern Pennines. Natural size.
-f
;rat
^
?
r o
t #
%% 4
t
/
Fig. 53. Acetocarmine preparations of the British species of Drjo/jfem sens. stiict. x 1000. a. D.aemula
(Ait.) O. Kuntze. n = \\. For explanatoiy diagram see Fig. 51. h. D. Villarsii (BeW.) Woynar from
Britain. n = 82. c. D. spinulosa (Mull.) Watt, somatic metaphase in a tapetal cell. 2n= 164. For
explanatoi-y diagram see Fig. 55. d. D. cristata {L.) A. Gray from Surrey. n = 82. For explanatory
diagram see Fig. 56. e. Diploid D. dilatata from Switzerland. For explanation see text. n = ^i.
J. Normal British form of D. dilatata (Hoffm.) A. Gray. w = 82.
D. c///a/-a/-a
THE GENUS DRYOPTERIS IN BRITAIN
ecologically to marshy woods. D. dilatata is relatively little bound to water, and in
many parts of Britain is both commoner and hardier than the Male Fern, and like that
species it has been recorded from every county and vice-county in the British Isles.
Some idea of the range of form exemplified by these three species and the hybrids
between them can perhaps be obtained from Fig. 54, which shows, not indeed the
whole plant, but the bottom pinna from a fully adult frond of the cytologically worked
specimens. The sources of material were as follows: D. spinulosa and D. dilatata have
been available in abundance from various localities in England, Scotland and Ireland,
D. spinulosa ^n - 164-
Fig. 55. Diagram to Fig. 53c. x 1500.
D. crist3f-3 n- 82
Fig. 56. Diagram to Fig. 53^. x 1500.
the particular specimens represented in Fig. 54a and e being from central Ireland.
D. cristata was at first only available in cultivation from a plant supplied to the late
Dr F. W. Stansfield by a dealer and believed to have come originally from Switzerland;
this has, however, latterly been supplemented by material from the Surrey plant kindly
supplied by the discoverer (Payne, 1939). Of the two hybrids the first (Fig. 54<:) came
from central Ireland, where it was found by Dr Praeger in company with the two
putative parent species. The second hybrid, D. uliginosa (Newman) Druce (Fig. 54^),
was supplied by the late Dr F. W. Stansfield from a continental specimen which,
owing to the excessive rarity of this type in Great Britain, has not yet been supple-
mented by one of British origin.
The cytological uniformity which prevails throughout most of the 'spinulosa' complex
makes it at first sight easy to deal with all the species collectively. All three taxonomic
69
THE GENUS DRTOPTERIS IN BRITAIN
species {D. dilatata, D. spinulosa and D. cristata), in British and in continental specimens,
have a regular meiotic process and identical chromosome numbers which are n = 82
for the reduced number and 2^2=164 for the sporophytic somatic tissues. Sample
photographs to illustrate these facts will be found in Figs. 53, 55, 56 and 71, of which Figs.
53 c and 55 give the unreduced number for D. spinulosa, Figs. 53^ and 56 show an aceto-
carmine preparation of the British D. cristata, and Fig. 53/ shows meiosis in D. dilatata,
so perfectly that a diagram is not required. A view of D. dilatata in a section will be
found in Fig. 7 1 c.
With regard to the hybrids, Fig. 57^ shows two spore mother cells from the hybrid
between D. dilatata and D. spinulosa at the first meiotic division in side view. The large
number of lagging unpaired chromosomes makes a striking contrast with the neat
%
,^^
.1.
'^ '/
Fig. 57. Meiosis in Dryopteris spinulosa hybrids from sections, x 1000.
a, D. uliginosa; b, D. spinulosa x D. dilatata, wild hybrid.
appearance of the parent species (Fig. 71c), and is sufficient confirmation of hybridity.
Since this particular hybrid is by no means rare, having been described from time to
time from many European countries and, in my experience, being not difficult to find
in Great Britain (I have examined specimens from England, Scotland and Ireland),
it may perhaps be of interest to field botanists to append a brief description of it.
The hybrid between D. spinulosa and D. dilatata is met with not uncommonly in
mixed populations of the two parent species such as may occur, characteristically, where
an old and previously swampy D. spinulosa habitat is beginning to dry out. The hybrid
(Fig. 54c) shows a mixture of the characters of the two parents very clearly. It has the
semi-erect rhizome and dark coloured scales of Z). dilatata, but a relatively narrow frond
with the bottom pinna shorter than the one above it as in D. spinulosa. It will often show
hybrid vigour by growing to a very great size, and the profuse stoloniferous branching
of the stock will, in an old plant, cause it to lack the neat shuttlecock-like form of its
parents and to present instead a rather shapeless but compact jumble of fronds. The
70
THE GENUS DRTOPTERIS IN BRITAIN
spores are commonly shrivelled, though it is not known whether a few might be viable.
The general appearance of mixed populations suggests strongly that this is probably
the case, and it is much to be desired that breeding work should be undertaken to
test this point, preferably after resynthesis of the hybrid from known parents.
The hybrid between D. spinulosa and D. cristata is better known than the one last
mentioned perhaps, paradoxically, in part because of its greater rarity, its frequency of
occurrence being necessarily hmited by that of Z). cristata, the scarcer of its two parents.
The morphological differences between D. cristata and D. spinulosa are greater than those
between the latter species and D. dilatata, so that it is not surprising to find that the
intermediate characters of the hybrid are sufficient to endow it with a marked indivi-
duality of its own. This led at an early stage to its recognition in the field as a distinct
morphological type to which the name D. uliginosa (Newman) Druce has been given.
Meiosis in D. uliginosa is shown in Fig. 570, and the close similarity between this and
the preceding hybrid is very evident.
The interpretation of the spinulosa complex on the basis of all this information would
therefore appear to be that we have here an old polyploid ' coenospecies ' * which
now contains three ' ecospecies ' * adapted to different degrees of waterlogged soil.
These have spread over a much larger geographical area than that so far colonized by
the relatively young Male Fern, and D. spinulosa, D. dilatata and D. cristata are wide-
spread in North America as well as in Europe and Asia, whereas the Male Fern itself
seems to be a characteristic occupant of the Old World only. The three ecospecies are,
however, still sufficiently akin to hybridize when they meet and such hybrids, though
highly sterile, nevertheless show sufficient proportion of pairing chromosomes to
demonstrate a fairly high degree of chromosome homology to be present among the
three ecospecies. Whether these themselves arose suddenly or by gradual accumulation
of mutational differences is, however, entirely unknown.
This is as far as the study of the D. spinulosa complex would have gone but for the
evidence of 'Z). remota\ to which problematical plant we may now turn. D. remota
is a putative species hybrid, thought to combine the characters of D. spinulosa and
D. Filix-mas. It is listed in the British Flora (e.g. Babington, 1922) on the basis of one
plant which was once found (by Huddard in 1867) in Brathay Wood on the shore of
Lake Windermere, and thereafter exterminated in the wild state though maintained and
multiplied by vegetative means m cultivation. It was much prized by the amateur fern
* We can scarcely do better here than to quote the definitions of terms given by Clausen, Keck and
Hiesey (1945), for the apphcation to experimental taxonomy of the ecological concepts originally intro-
duced by Turesson. The chief of these are :
'(i) Ecotype (Turesson, 1922a, 19226): all the members of a species that are fitted to survive in a
particular kind of environment within the total range of the species.
(2) Ecospecies (Turesson, 19226, 1929): all the ecotypes genetically so related that they are able to
exchange genes freely without loss of fertility or vigour in the offspring.
(3) Coenospecies (Turesson, 19226, 1929): all the ecospecies so related that they may exchange genes
among themselves to a limited extent through hybridization.'
It will probably already be apparent that while knowledge of the Pteridophyta is only rarely sufficiently
advanced to deal in ecotypes, both of the other concepts will be found from time to time to coincide with
the taxonomic species.
71
THE GENUS DRTOPTERIS IN BRITAIN
collectors of the last century, owing to which fortunate circumstance it is still possible
to obtain portions of it in the living state in old collections, and I was most fortunate in
being given access to a very large and ancient specimen of authentic Windermere
origin in the garden of the late Dr F. W. Stansfield of Reading. In Babington's Manual
(1922) 'L. remota Moore' is listed as a species, with the
possibility that it might be a hybrid indicated only in
brackets. The complete sterility of the plant when spores
are sown, however, has always been known to British
pteridologists, who have been unanimous in their opinion
that it is a hybrid, though the attribution of parentage
has varied somewhat. D. Filix-mas, D. spinulosa, D. dilatata
and D. rigida have all been named from time to time in
this connexion, though the general consensus of opinion
has been in favour of the combination D. Filix-mas x D.
spinulosa. Some of the reasons for this diagnosis may per-
haps be judged by examination of the pinna shown in
Fig. 58.
Cytological examination of the Windermere plant
has strongly confirmed the diagnosis of hybridity with-
out thereby throwing any further light on the nature of
the parental species, a fact which need cause no surprise,
since all those listed have identical chromosome numbers
and could therefore not be distinguished cytologically.
As expected, the chromosome number found in the
roots of the Windermere plant is of the order of 160
(Fig. 59^) (that of both putative parents being of the
same order, i.e. 2/2=164). Meiosis in the Windermere
plant is extremely irregular (Fig. 59 a), a circumstance
which fully explains the sterility of the spores.
Since a single plant incapable of reproduction and
demonstrably a hybrid cannot possibly be accepted as a
species on any definition of that word, these findings at
once indicate that ' Lastrea remota Moore' is not the same thing as the continental
material to which the specific epithet remotum was first applied. This* was a plant first
found by A. Braun near Geroldsau in Baden and described by him in 1 843 as Aspidium
rigidum j3 remotum, though subsequently raised to the rank of a species in 1850 under the
name of ^. remotum A. Braun. This 'species' is fertile from spores but apogamous, and
when investigated by Dopp in 1932 a chromosome number was found which approxi-
mates to the triploid condition in this circle of affinity (Dopp's actual number was
reported as c. 130; the triploid condition itself is now known to be 123). Morpho-
logically A. remotum A. Braun resembles the Windermere plant in that it looks like a
hybrid between the same two parent species, namely, Dryopteris Filix-mas and D. spinu-
losa, though its triploid nature makes its exact derivation more problematical, since
* Cf. Praeger (igog).
Fig. 58. Lowest pinna from a
large frond of an offset from the
original 'Lastrea remota Moore'
from Windermere. Natural size.
72
THE GENUS DRYOPTERIS IN BRITAIN
both its suggested parents are now known to be tetraploids. By virtue of its apogamous
reproduction it is not a single plant but forms local populations in several parts of
Germany, Alsace and Silesia, while a variant which has spread to Switzerland has
there been designated Aspidium remotumva.T. subalpina Borbas,* or sometimes Dryopteris
Borbasii Litard.
I have not seen any material of the original Aspidium remotum A. Braun, but a plant
purporting to be var. subalpina from Switzerland was kindly presented to me by the
late Dr F. W. Stansfield, the lowermost pinna of which is represented in Fig. 60. This
agrees closely with Luerssen's Fig. 145 (Rabenhorst's Flora, 1889) of ^. remotum A.Br.,
and the importance of giving varietal status to the Swiss material may perhaps be
doubted. Be that as it may, my specimen of 'subalpina' agrees with Dopp's remota in
being apogamous and approximately triploid.
Fig. 59. The Windermere 'Lastrea remota\ from sections. X 1000. a. Meiosis. b. Two extreme focal
levels through a metaphase plate in a root with approximately 164 chromosomes, for comparison
with Fig. 61.
Though Dryopteris remota in the continental sense cannot be claimed for Britain on
the evidence of the Windermere plant discussed above, there are nevertheless two other
records of single plants each found once and exterminated in the wild state by the act
of collection though maintained in cultivation, which place the matter in a rather
different light. These are ' Lastrea Boydii\ collected on the shore of Loch Lomond
at the end of the last century (see Stansfield, 1934; von Tavel, 1934) and at first
identified as L. remota from a general resemblance to the Windermere plant; and
another specimen found in central Ireland by Praeger in 1898 (see Praeger, 1909) and
also identified as L. remota. Both these finds have been available to me as spore descen-
dants of the original plants, 'Dryopteris Boydii' being presented to me by the late Dr
Stansfield and the Irish remota having been obtained from Dr Praeger in 1935. Pinnae
of both these plants are shown in Fig. 60, and their general resemblance to the con-
tinental material is at once apparent. Cytologically (Fig. 61) these plants also resemble
the continental material in being apogamous and triploid.
* Cf. Luerssen (1889).
73
THE GENUS DRTOPTERIS IN BRITAIN
It therefore seems probable that D. remota in the continental sense, or at least plants
very like it, do occur sporadically in Britain, though the evidence suggests that they
D. nemo/- a ( /re/and)
Bo^dii
Suhcr/pina
Fig. 60. The lowest pinna from plants of different age of apogamous triploids
referable to Dryopteris remota (A.Br.) in the continental sense. Natural size.
have not yet become established as local populations. The question of their origin is
therefore a matter of interest. They could perhaps arrive in Britain as stray spores
from the Continent. If this is so, however, it is perhaps remarkable that they should be
74 .
THE GENUS DRTOPTERIS IN BRITAIN
found first in relatively remote parts of the British Isles (central Ireland and Scotland)
and not, so far, in the more accessible parts of England or Wales. The other alternative
is that they have been formed de novo in each locality by an act of hybridization, but
here we are confronted with the dilemma that all the species with which a parental
relation has been suspected [D. spinulosa, D. dilatata, D. rigida, D. Filix-mas) are tetra-
ploids and could therefore not give rise at once to a triploid by a simple cross.
A possible solution to this problem was detected in the summer of 1948 by the dis-
covery that a diploid form of D. dilatata exists on the Continent and, from its morpho-
logical characters, probably also in Great Britain, though a plant of British origin has
not yet been confirmed cytologically. Fig. 62 shows a complete frond, slightly reduced
but otherwise characteristic of a population of small individuals found growing m
profusion on the north face of a sheltered gully above the tree line on the frontier
b
Fig. 61. The cytology oi ' Dryopteris remota' from Ireland, for comparison with that of the Windermere
specimen. For description see text. From sections, x 1000. a, b. Two different focal levels
through the same group of mother cells showing a polar and a side view. Note the extreme
regularity and the large number of the chromosomes, c. Two focal levels of a mitotic metaphase
plate in a root. Note the lower chromosome number in comparison with Fig. 59^.
between Norway and Sweden at Storlien in Jamtland. Its oval outline is distinctly
narrower than is normally shown by lowland forms of D. dilatata in Britain, and the
shape of the lowest pinna bears some resemblance to D. spinulosa (cf Fig. 54^); it has,
however, quite unmistakably the dark central streak to the scales [ramenta) which
is otherwise diagnostic of D. dilatata. Fig. 63 shows the central portion, natural size,
of a larger specimen found in a wood at sea-level at Trondheim in Norway, and Fig. 64
shows the lowest pinna only of a still larger plant of the same general character which
came from a high akitude at Arolla in Switzerland. All these plants agree in having
41 chromosomes instead of 82 in their spores, a feature which is perhaps sufficiently
demonstrated by a comparison between Fig. 53^, p. 67, from the Swiss plant with Fig. 53/
of normal British D. dilatata placed immediately beside it. All agree also in their
more finely cut pinnation, as may be seen by comparing any of the three Figs. 62-64
with Fig. 54. This character is mentioned from time to time in the early British
literature in relation to various rather ill-specified 'varieties' of D. dilatata, which is one
75
Fig. 62. Whole frond of a small, high mountain form, of the new diploid oi Dryopteris dilatata (see text)
from Storlien, Jamtland, Sweden. | natural size.
Fig. 63. Base of the leaf blade of a lowland form of the new diploid of Dryopteris dilatata
from Trondheim, Norway. Natural size.
THE GENUS DRYOPTERIS IN BRITAIN
reason for believing that the plant is likely to be found here though not yet diagnosed.
Further information about diagnostic characters, genetical affinities, geographical
distribution and appropriate name must, however, await further study. Enough has
Fig. 64. Lowest pinna of a very large plant of the new diploid of Dryopteris dilatata
from the Alps. Natural size.
nevertheless perhaps been said to explain why taxonomists working on the alpine and
subarctic forms of the spinulosa complex have been less satisfied with the simple sub-
division into D. spinulosa, D. dilatata and D. cristata than have botanists working on the
lowland populations of more equable latitudes.
78
THE GENUS DRYOPTERIS IN BRITAIN
We have also now a new possibility for the parentage of Z). remota. If the new diploid
were to hybridize with D. Filix-mas, or another of the suggested species of appropriate
morphology, a triploid would be formed at once which might or might not be apogamous
but which would be expected to show much the same mixtiue of characters that we find
in D. remota. Attempts to synthesize such a hybrid have already been begun though they
will take several years to mature. If they are successful a very long-standing problem
in the European flora will have been solved.
Leaving these problems aside, we come next to two very distinct and well-known
species, D. Oreopteris Ehrh., the Sweet Scented Mountain Fern, and the Marsh Fern,
D. Thelypteris (L.) A. Gray. In habitat and in habit these two species are about as far
removed from each other as any in the genus. The Mountain Fern is never found far
from hills; it is of the ordinary Lastrea type, growing abundantly in exposed positions
I
^ *
* y ♦ •
Fig. 65. Explanatory diagram to Fig. 66. x 1500.
up to 1680 ft. in England, 2000 ft. in Ireland and 2900 ft. in Scotland, sometimes
mixed with Dryopteris abbreviata, D. Borreri or D. dilatata at the lower levels but eventually
outstripping these as it ascends. The Marsh Fern has a creeping habit and grows sub-
merged in the wettest of lowland bogs to which it is absolutely confined. In nature these
two species probably never meet under present conditions, and that they are ancient
is indicated by a geographical distribution which includes America as well as Europe.
To a cytologist they show, however, several striking points of resemblance. Although
tlie Marsh Fern has a curiously inverted periodicity in putting up its fertile leaves after
its sterile leaves, at the end of July, the sori share with those of the Mountain Fern the
peculiarity that they are placed very near to the edge of the pinnules, and though an
indusium is present, the main protection for the sporangia at the stage of full meiosis
is the recurved margin of the leaf They also differ very markedly in chromosome
number from all other British species of the genus.
Fig. 71a represents meiosis in D. Oreopteris as it appears in sections, and Figs. 65 and 66
show a similar stage in a squash preparation in which the number n = 34 is very clearly
displayed.
Figs. 67 and 68 represent a squash preparation of the Marsh Fern, D. Thelypteris.
The number appears not to be 34 but still less is it 41. In this species, as nearly as can
be determined, n = 35.
79
THE GENUS DRTOPTERIS IN BRITAIN
The interest of finding an entirely new chromosome number in each of these two
species is enhanced by two further circumstances. It has aheady been seen that the
gametic number of 41 is fundamental to the D. Filix-mas complex, the D. spinulosa
complex, to D. aemula and D. rigida, but as will be shown in detail in the next chapter,
\
•
f» •* «
M
%.
0
«
4
9
%•
•%
m
m
irm
^^^^ ^
Fig. 66. Meiosis in Dryopteris Oreopteris (Ehrh.) Max. Fig. 67. Meios'is in Dryopteris Thelypteris
showing « = 34, permanent acetocarmine. x 1000. (L.) A. Gray showing « = 35, per-
For explanatory diagram see Fig. 65. manent acetocarmine. x 1000. For
explanatory diagram see Fig. 68.
^£ht
S^
D. Thelypteris n-35
Fig. 68. Explanatory diagram to Fig. 67. x 1500.
it is equally so to a number of other related genera, notably Polystichum, Cyrtomium and
perhaps Woodsia. It must therefore be an ancient character which may even antedate
the evolutionary separation of Dryopteris. It would, however, be a mistake to suppose
on this account that the numbers 34 and 35 are merely recent developments from
this. The inference should probably be that the generic boundaries have here been
wrongly drawn, and some very instructive comments suggesting this have been made
from time to time by taxonomists. Thus in 1920 Christensen wrote: 'As I have tried
80
THE GENUS DRTOPTERIS IN BRITAIN
to prove in my earlier papers on Dryopteris, that large genus of ferns may be divided
into a number of well-defined subgenera, and in the first part of the present monograph
I have referred the tropical American species . . .to ten subgenera. Since that part was
published I have examined about 3000 specimens of species with more divided leaves,
and these later studies have confirmed that my classification is a natural one. I have
no doubt that those species which are grouped together within the same subgenus
belong together genetically, while, on the other hand, they are very remotely
related to species belonging to other subgenera. I am convinced that most, if not
all, of the defined subgenera really are good genera such as genera are commonly
understood.'
A further stage in the subdivision of Dryopteris on these lines is contained in Ver-
doorn's Manual of Pteridology published in 1938, in which Christensen himself separates
a much-reduced genus Dryopteris, with D. Filix-mas as its type species, into a separate
tribe from the genus Thelypteris with the Marsh Fern ( T. palustris) as its type species.
With the Marsh Fern is placed the Mountain Fern under the name T. Oreopteris, and
also the Beech Fern, which will be dealt with below. In Christensen's opinion (Ver-
doorn's Manual, p. 543) the old genus contains 'at least two phyletic lines which
have arrived at, or perhaps preserved the same soral condition, but as to anatomical
structure. . .have followed different lines'. A still more extreme view is expressed by
Holttum, whose Revised Classification of the Leptosporangiate Ferns (1947) appeared during
the writing of this chapter. In this he remarks that 'it seemed to me that too little stress
had been laid on the differences of the Thelypteris group of genera from the Dryopteris
group'. In Holttum's view Thelypteris is likely to have been descended from a Cya-
theaceous or Gleicheniaceous ancestor, as Bower believed to be the case with the whole
'genus', but Dryopteris in the narrow sense he would relate to Dennstaedtia. Copeland's
views (1947) are not dissimilar though his nomenclature is somewhat dififerent.
These expressions of opinion are perhaps enough to warn us against wasting time at
this stage in premature discussions as to what nuclear condition can have been primitive
in 'Dryopteris' as a whole, and we need only record the fact that the Marsh Fern and
the Mountain Fern are as diflTerent from all the species previously considered in their
chromosome numbers as they appear to be in their other characters.
Christensen's mention of the Beech Fern as a possible relative of Thelypteris rather
than of Dryopteris in the narrow sense may serve to introduce the last three species with
which this chapter will be concerned. These are, as explained at the beginning of the
chapter, the Beech Fern, the Oak Fern and the Limestone Polypody. So confused is
the Latin nomenclature of these very distinct and well-known species that for once the
English names may be preferred. The naked sorus, which they all share, has not
confused the specific identity of any of them, but it has led to considerable doubt
regarding the generic affinities which are by no means yet resolved. The old solution,
to put them all in the genus Polypodium along with the common Polypody itself
{P. vulgar e), has by common consent now been abandoned. To assimilate them all into
Dryopteris is, however, also unsatisfactory now that the polyphyletic nature of that
•genus' has been made clear. The other akernatives can perhaps better be discussed
after the cytology has been examined.
81 6
MFC
THE GENUS DRYOPTERIS IN BRITAIN
The chromosomes of the Beech Fern {Phegopteris) are shown in some detail in Figs. 69-
71. In Fig. 71/meiosis is seen in a section, and the somewhat smaller size of the
chromosomes than in the species previously examined may be seen. The chromosome
number in the metaphase plate shown is of the order of 90 pairs, and this number can
:• ••:r?>
1 ^
1
/
A
%♦ »
♦
♦
■ 4
*«
A
&
•
4
1
>
*
t
i
u_
^'
n
Fig. 69. Meiosis in the Beech Fern {Phegopteris). Permanent acetocarmine preparations, a, b. From
a British specimen, two cells in the same preparation, x 500. For explanatory diagram of a see
Fig. 70. c. A Scandinavian specimen from Storlien, Jamtland, Sweden, showing 90 chromo-
some pairs with exceptional clarity.
be seen again in the squash preparations of Fig. 69, with one explanatory diagram in
Fig. 70 fl. The large cell of Fig. 69^ is a giant mother cell from the same sorus as Fig. 69 a
showing twice this number, an occurrence which will be discussed in greater detail in
Chapter 9.
A section through a sorus of the Beech Fern, however, shows at once that the majority
of the sporangia have only 8 spore mother cells, instead of the customary 16, a condition
82
THE GENUS DRTOPTERIS IN BRITAIN
which was noticed in Dryopteris Borreri (Chapter 4) as indicative of obhgate apogamy.
In the Beech Fern its significance is no less and a chromosome count in a root, as
indicated by Figs. 70^ and 71^, also shows about 90 chromosomes. The meiotic
process, for all its apparent regularity, is, therefore, ineffectual as a means of changing
the nuclear content, and both morphological generations possess the same number
of chromosomes. By sowing the spores it is not difficult to demonstrate that the
resulting prothalli are indeed apogamous as anticipated.
t
^ Phe^opf-en's
^^ (Bazch F(zrn) ^"\-90
t
«•« "*
i ' ^*M
|fV ***^*' Sfjl/^
- ♦ \ %t<^
Fig. 70. Explanatory diagram to Figs. 6ga and yie.
The detection of obligate apogamy yet once more is a matter of some interest. We
have met it so far in D. Borreri and D. remota, but that there can be any close relation-
ship between either of these and the Beech Fern is quite out of the question, and the
polyphyletic origin of apogamy must be accepted as irrefutable. With regard to the
possible parentage or phyletic affinities of the Beech Fern we are wholly in doubt.
The chromosome number is so unlike that of any other known species of either Dryopteris
or Thelypteris that pending further information it seems better to suspend judgement by
placing the Beech Fern in a separate genus Phegopteris as has sometimes been done,
rather than to prejudge the position by following Christensen in uniting it with
Thelypteris. Its name under this treatment should then be Phegopteris polypodioides Fee.
A similar procedure may also be advised for the remaining two species. The Oak
Fern and the Limestone Polypody are much alike cytologically and have a normal
83 6-2
THE GENUS DRYOPTERIS IN BRITAIN
sexual life history. The chromosomes agree with those of Phegopteris in their small
size, a fact which is demonstrated by comparing Fig. 7 1 d from a section of the Oak Fern
with other photographs on this page. The number, however, is different. Owing to the
*■ V
tCLyASgig'.
•
<0^*
#
4i^^ .
d e f
Fig. 71. Cytology of some species o{ Dryopteris in the wide sense seen in sections, x 1000. a. Meiosis
in D. Oreopteris (Ehrh.) Max. ti = 34. For further details see Figs. 65 and 66. b. Meiosis in D. aemula
(Ait.) O. Kuntze. n = ^i. For further details see Figs. 53a and 51. c. Meiosis in D. dilatata
(Hoffm.) A.Gray. n = 82. For further details see Fig. 53/. d. Meiosis 'miheOakYern{Gymnocarpium).
n = 80. For further details see Figs. 72 and 73. Note small size of the chromosomes, e. Mitosis in a
root of the Beech Fern (P/i^^o/)/mi). 2^ = 90. For explanatory diagram see Fig. 70^. f. Meiosis in
the Beech Fern {Phegopteris) with the same number of chromosome pairs as single chromosomes in
the root. For explanation see text.
small size of the chromosome it has proved unusually difficult to determine their number
with absolute accuracy. In both species n is of the order of 80, and in the Oak Fern it
appears to be exactly this number (Figs. 72c, 73). It therefore seems possible that these
two species when better known may prove not to be identical with other European
84
THE GENUS DRTOPTERIS IN BRITAIN
members of the old genus Dryopteris, and their separation, suggested first by Newman
in 1 85 1, into an independent genus Gymnocarpium is perhaps desirable. If this were
done their names should be G. Dryopteris (L.) Newman and G. Robertianum (Hoffm.)
«
.1
^
v
^
r
-ytil
•.*
-»,
« <
?. C fc
•*
^p^
'^
•
<* If % * • ^
Fig. 72. Meiosis in Gymnocarpium, permanent acetocarmine. x 1000. a. The 'Oak Fern' in Britain.
b. The 'Limestone Polypody' in Britain, not countable with certainty but not less than 80 or more
than 84 pairs, c. The Oak Fern from Storlien, Jamtland, Sweden, showing 80 pairs of chromosomes
with exceptional clarity. For explanatory diagram see Fig. 73.
Newman, and it may be said in passing that such a procedure has been advocated on
morphological grounds by many recent writers, e.g. Ching, Christensen and Holttum.
Summing up the information for the British species of 'Dryopteris' it may be stated
that not one but probably at least four distinct genera are actually represented, some
of which, notably Dryopteris in the narrow sense and Thelypteris, seem to have come from
85
THE GENUS DRTOPTERIS IN BRITAIN
widely different sources and to owe their present resemblance to parallel evolution.
Within the curtailed genus Dryopteris polyploidy and hybridization are rife; the other
three genera are still too few in analysed species for conclusions to be drawn. Apogamy
has arisen repeatedly, both in the restricted genus Dryopteris and in the relatively un-
related Beech Fern, thereby adding a very clear example of the polyphyletic origin
of a fairly complicated set of characters.
V
Oak Fern (Gt/mnocarpium) n = 80
Fig. 73. Explanatory diagram to Fig. 72 c. x 1500.
SUMMARY
Since the above statement recapitulates most of the points of general interest raised in
the chapter a formal summary may perhaps be replaced by a simple hst in which the
cytological facts of this chapter and the last are assembled. The specific and generic
subdivisions follow the conclusions given in the text of both chapters, but special
attention should perhaps be drawn to the newly discovered diploid forms of D. dilatata,
PP- 75-785 ^^^ ^- Villarsii [D. rigida), p. 65, both of which deserve further study.
86
THE GENUS DRYOPTERIS IN BRITAIN
Cytology of the genus
Dryopteris sensu lato in Britain and West Europe
Name
2n
n
Reproduction
Status
UTyopieits.
D. Filix-mas (L.) Schott sens. strict.
164
82
Sexual
AUotetraploid species
D. abbreviata (Lam. & DC.) Newman
82
41
55
Diploid species
D. Borreri Newman
82
82
Apogamous
Diploid form
123
123
>)
Triploid form
164
164
>>
Tetraploid hybrid
205
205
J)
Pentaploid hybrid
D. ae/nula (Ait.) O. Kuntze
82
41
Sexual
Diploid species
D. Villarsii Woynar ( = D. rigida (Hoffm.)
Underw.) :
British
164
82
j>
T^. , ■ , Status undecided
Swiss
82
41
3>
Diploid III! Ill
D. spinulosa (Mull.) Watt
164
82
yj
Tetraploid species
D. cristata (L.) A. Gray
164
82
s>
>> >>
D. dilatata (Hoffm.) A. Gray
164
82
»
Tetraploid 1
'Z). dilatata':
Swiss
,
41
JJ
■Status imdecided
Swedish
.
41
J>
Diploid
Norwegian
82
•
>>
-^
D. uliginosa (Newm.) Druce
164
.
Sterile
Species hybrid
D. spinulosa X D. dilatata
164
.
jj
)> >>
'Z). remota Moore', Windermere
c. 164
.
>>
)) J)
D. remota var. subalpina Borbas
{ =
c. 120
prob. li
3)
c. 120
Apogamous
Triploid species-hybrid
'D. remoia Boydii\ Scotland
( =
c. 120
prob. i:
=3)
c. 120
it
»i >>
D. remota (A.Br.) Hayek, Ireland
c. 120
c. 120
»
» »>
( =
prob. 12
•3)
Thelypteris:
T. palustris Schott
70
35
Sexual
Diploid species
r. Oreopteris (Ehrh.) Sloss.
68
34
>>
>> j>
Gymnocarpium:
G. Dryopteris (L.) Newman
•
80
j>
Species
G. Robertianum (Hoffm.) Newman
•
c. 80
(=80-84)
»
j>
Phegopteris:
P. polypodioides Fee
90
90
Apogamous
»
87
CHAPTER 6
THE OTHER BRITISH FERNS: POLTSTICHUM,
ATHTRIUM, ASPLEJVIUM, CETERACH
The British fern flora, a quarter of which has now been passed in review, is less than a
hundredth part of the ferns of the world, yet it is fortunately sufficiently varied to serve
as a qualitative sample of ferns in general provided always that the whole of it is studied.*
This can perhaps best be shown by a diagrammatic reproduction of Bower's phyletic
views (Fig. 74), from which it will be seen that of the six major groups into which the
leptosporangiate ferns are divided only one, the 'Davallioids', is wholly without a
British representative. It should perhaps be pointed out that only the higher ferns are
included in this scheme, the more primitive ones comprising the Eusporangiatae,
Hymenophyllaceae and the Osmundaceae being more conveniently deferred to the
end of the book. All the other British genera and every British species in each will,
however, now be reviewed in this and the two following chapters, and if again the
narrative at times resembles a catalogue apology can no longer be made for it is of the
essence of a sample that it should be within its hmits complete but composed of varied
and, if necessary, unconnected elements.
It may, however, be helpful in maintaining a thread of continuity with the two
preceding chapters if we begin our wider survey with the remaining British genera of the
'Dryopteroid' affinity. This, as a glance at the diagram will make clear, contains four
other genera and two doubtful ones in addition to ' Dryopteris\ and to the first of these,
namely Polys tichum, we may now turn.
There are three British species of Polystichum, illustrated as fully as the size of the
page permits in Figs. 75-77. The first of these, P. Lonchitis (L.) Roth, the Holly Fern,
is rare in the British Isles except on some of the richer Scottish mountains. It is usually a
plant of high altitudes save for a few localities, such as the hmestone ' Pavement ' areas
of Yorkshire. Once seen in the fully mature condition it cannot be confused with any-
thing else, though young plants of P. aculeatum are sometimes mistaken for it by those
unfamiliar with the true Holly Fern. Though rarely abundant in Britain it is both wide-
spread and often prolific in many other European countries, having a total range which
* The importance of this proviso will probably become more obvious at a later stage in the book,
but it should perhaps be pointed out at once that completeness of a sample within the parameter chosen
(in this case the geographical limits of the flora of Britain) is as necessary as any other consideration to
its claim to be fairly representative. Any selection within the sample leading to deletion of certain ele-
ments for extraneous reasons such as horticultural or cytological convenience to the observer will
necessarily falsify the picture by over-simplification.
The impossibility of obtaining truly random sampling of the world's vegetation in a botanic garden
is one of the many reasons for believing that a complete study of a local small flora will be far more
informative than the comparison of a greater number of miscellaneous species of unknown origin which
might happen to be available from horticultural sources.
88
.1
THE OTHER BRITISH FERNS— POLl^STICHUM, ATHTRIUM, CETERACH
extends from the Himalayas on one side to Greenland and eastern North America on
the other. Both the other British species are commoner than the Holly Fern in our flora.
P. angular e Presl [P. setiferum (Forsk.) Woynar),* the Soft Prickly Shield Fern, is a
woodland plant more frequent in Ireland and the south of Great Britain than in the
north and lacking the preference for limestone shown by the other two species. When
growing luxuriantly as it often does in parts of Devon it exceeds the Male Fern in size,
Dai/o//jo/ds
D/cksoniacece
Pteroids
Osmundaceoe
P/sg/opuriaceoe Gymnogrammoids
B/echnoids
eoe
\
\
\
DryopZ-ero/ds
Dipf-eroids
{
No British
representative
{
Ptendium
Cryptogrsmme
Adiantum
Anogramme
Blechnum
Scolopendrium
( Dryopteris
Po/ysticham
Athyrium
< Asptenium
Ceteracti
mods/a (n
Cystopterts (?)
Pofgpodium (?)
Fig. 74. Phylogenetic affinities of the principal genera of British ferns (right-hand column)
redrawn after Bower (1929, 1935).
and as in that species the fronds die down in winter except in a very mild climate.
P. aculeatum (L.)Roth, the Prickly Shield Fern, is of a much tougher texture and, like the
Holly Fern, is normally evergreen in all climates in which it occurs. It is commoner in
the north of Great Britain than the south, and is characteristically a plant of rocky
ground though it does not usually occur as high on mountains as the Holly Fern. The
geographical ranges of the last two species outside Great Britain and Europe are not
clearly known owing to their confusion with each other and with non-European species.
* According to the International Rules the valid name for this species should now be P. setiferum.
The retension of the older name of P. angulare for the purposes of this chapter and again in Chapter 9
is merely a temporary expedient for the sake of consistency with the principles of nomenclature explained
in the Preface.
89
i
^"ivii^i^^
Fig. 75. Part of an adult fertile
frond of the Holly Fern {Polysti-
chum Lonchitis (L.) Roth) from
Scotland. Natural size.
Fig. 76. Part of an adult fertile frond of Polystichum angulare
Presl from Devon. Natural size.
90
Fig.
(L.
77. An adult fertile (rond of Polystichum aculeatum
Roth from Ingleborough, Yorks. Natural size.
9.1
THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH
That they are distinct and vaHd species is, however, now the prevailing opinion among
systematists, ahhough much discussion of this will be found in older Floras, owing
probably to the fact that hybrids of intermediate morphology can sometimes be found.
Cytological observations have been made on British and on
continental specimens of all three species. The Holly Fern,
P. Lonchitis, has been examined from Scotland, Ireland and
Switzerland with identical results; as shown in Figs. 78 and
jgc the reduced chromosome number is « = 41. This number
was found again in P. angulare from Devon, the Lake District
and north Italy, and is very clearly demonstrated in Fig. 79 a.
On the other hand, P. aculeatum from the Lake District,
Yorkshire and Switzerland has twice this number, as may be
seen in Figs. 79^, 79^ and 80.
P./onchih's n=4l
Fig. 78. Explanatoiy dia
gram to Fig. 79c. x 2000.
• ^
• . *j
* *•
¥
«
t^
^
^i»'
Fig.
79. Meiosis in British species of Polystichum. x 1000. a. Permanent acetocarmine squash of
P. angulare Presl. n — 4.1. b. Permanent acetocarmine squash of P. aculeatum (L.) Roth. « = 82.
For explanatory diagram see Fig. 80. c. Section of P. Lonchitis (L.) Roth. 71 = 41. For explanatory
diagram see Fig. 78. d. Section of P. angulare. n-J^i; cf. a. e. Section of P. aculeatum. 7j = 82;
cf. ^.
Further information about this genus will be found in Chapter 9, and for the present
the only conclusions which need be drawn from these facts are that the close affinity
between Polystichum and the Male Fern section o{ Dryopteris is strongly confirmed by the
92
THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH
identity of chromosome numbers, as is the vaHdity of the specific distinctness of Poly-
stichum aculeatum from P. angulare.
The second genus thought to be closely related to the Male Fern by Bower is Athyrium,
the Lady Fern. A. Filix-femina (L.) Roth, the Lady Fern itself, needs little if any intro-
duction. As its name implies it resembles the Male Fern in a number of ways, notably
in size and in general habit, but the more delicately cut up pinnules as well as the
elongated indusium will at once distinguish it. It is hardy and abundant throughout
the British Isles and has probably provided more of the monstrous and peculiar forms
beloved of collectors than has any other British species. For this reason it is a common-
place of gardens, though often it must be admitted in a bizarre condition which bears
little resemblance to the normal wild species, which is on the whole surprisingly con-
/? dcu/<Z3/-um n = 82
Fig. 80. Explanatory diagram to Fig. I^h. x 1500.
stant in appearance. A characteristic difference from the Male Fern is the very tender
foliage which dies down sooner in autumn than is usual in any of our other large hardy
ferns.
One of the more important taxonomic characters for the classification of the Lady
Fern is the shape of the indusium which resembles so much that of the Spleenworts
[Asplenium) that the Lady Fern was for long included in that genus. This has, however,
often aroused protests in the handbooks for fern collectors, e.g. Newman. The justice
of these views has at last been recognized by the separation of Athyrium as a genus
distinct from Asplenium and with a closer connexion with Dryopteris (sens.lat.).
The cytology of the Lady Fern has been examined in specimens from Scotland, the
Lake District and Yorkshire. At first the resemblance to some of the diploid species
previously studied is so close that identity could readily be assumed. Such an assump-
tion would, however, probably not be correct. After repeated study from which a
decision has been difficuh, my personal interpretation of Athyrium Filix-femina is that
the haploid chromosome number (see Figs. 81 and 82) is not 41 but 40. This, if
93
THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH
confirmed, and confirmation from another source is very desirable, will not necessarily
invalidate the suggestion of a derivation from Dryopteris sens.lat. (especially in view of
the findings for Gymnocarpium in Chapter 5), but ^^
might merely indicate that the relationship to ^ ^Jt
the Male Fern itself is less close than between W'^ *#^ ^
that species and Polystichum, a conclusion which ^^
few systematists would deny. Of more impor- ^^L ^%^ % ^
tance than this is the strong confirmation which ^|^
this chromosome number provides of a lack of *^
close affinity with the next genus Asplenium.
iU ^^
Before discussing this aspect, however, it will ^^l ^'wk!^
first be necessary to complete our study of the ^ • i^ ^ ^
genus Athyrium by consideration of two other C^ J*^
species which are generally, though not always,
included in it. ^^
The other two species oi Athyrium present in f
Britain are far less familiar than is the Lady
Fern, since both are confined to the Scottish ^^ ^_
mountains and one is so very limited even in n=^ ^
Scotland that few living botanists have ever Athurium F/7/x fem'ina
collected it, and there is therefore still doubt ^. ^ ^ , ,. t^- r,
rig. 01. Explanatory diagram to rig. 02a.
as to whether it is really a species or only a ^ ^
local mutant. Assuming for the moment that
they are both species, the two in question are A. alpestre (Hoppe) Rylands and A. fiexile
(Newman) Syme, both differing from the Lady Fern by the absence or early abortion of
a*
^ •
J
V
>* ^^
^ w
Fig. 82. The British species of Athyrium. x 1000. a. Meiosis in a permanent acetocarmine mount of
A. Filix-femina (L.) Roth. ^ = 40. For explanatory diagram see Fig. 81. b. Two focal levels of
a root section showing a somatic chromosome count, in A. alpestre (Hoppe) Rylands. 2« = 8o.
For explanatory diagram see Fig. 84. c. Fresh acetocarmine preparation of A. flexile (Newman)
Syme. ^ = 40. For explanatory diagram see Fig. 88.
94
THE OTHER BRITISH FERKS—POLTSTICHUM, ATHYRIUM, CETERACH
the indusium and therefore, as in the comparable case of Gymnocarpium and Phegopteris
(Chapter 5), formerly classed as Polypodium.
Athyrium alpestre (Fig. 83) is superficially very similar to the Lady Fern proper when
well developed, though it is generally somewhat smaller and in cultivation matures its
fronds much earher. In Scotland it occurs at an altitude shared by the Holly Fern,
and like that species it is less restricted in range on the Continent than with us, being
locally abundant, at suitable altitudes, in most of the mountain ranges of Europe and
Fig. 83. Pinnae of Athyrium alpestre (Hoppe) Rylands, from two different plants to show range of
morphology in this species on Ben Alder, locality o^ A. flexile. The left-hand plant is probably the
variety obtusatum Syme. Natural size.
in parts of America. Cytologically it appears to resemble the Lady Fern, although my
study of it is imperfect. I have had several Scottish plants in cultivation for many years,
but they are fertile so infrequently that I have not yet seen
meiosis. From a root-tip count represented by Figs. 82^ and
84 the somatic number, however, appears to be 2n = 80.
The third species o{ Athyrium, A. flexile (Newm.) Syme, is a
very remarkable and interesting little plant which would well
repay genetical investigation. Its claim to specific rather
than to varietal status can only finally be settled after
breeding experiments with its nearest relatives have been
carried out, but on morphology alone it would probably
long ago have been accepted as a species but for the circum-
stance that the only known locality in which it now occurs in
any quantity is so remote and difficult of access that very few
botanists have ever visited it. The locality in question is the
great corrie on the north face of Ben Alder in west Inverness-
shire, where it was said to be abundant by Professor
J. H. Balfour in 1867 and where in 1946 it was still plentiful.
The only other station in which it has ever been recorded
in quantity is Glen Prosen, Forfarshire, where the t>'pe
specimens (cf. Fig. 85) were first found by Backhouse in
1853. A. flexile may still exist in Glen Prosen, but it has not recently been seen there.
The following notes have, however, been supplied from Ben Alder by my colleague,
Dr Sledge, who in August 1946 camped at the foot of the mountain (which is 12 miles
away from the nearest house) in order to search for the plant and who succeeded in
bringing back a number of living specimens (cf. Fig. 86) in excellent condition which
have since been maintained in cultivation.
95
• • • •
•• :'•
2/1' 80
/I. a/pestre
Fig. 84. Explanatory
. diagram to Fig. 82 h.
X 2000.
THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH
'Athyrium flexile grows intermixed with A. alpestre amongst rocks in the north corrie of
Ben Alder from 2750 to 3400 ft. Matm^e plants of A. flexile are readily distinguishable by
their much smaller size — the fronds rarely exceed 6 in. in length — by the more distant
and usually deflexed pinnae with narrow-based pinnules and the distinctly narrower
FLEXILE LADY FERN, {natural size of a large plant).
Fig. 85. Newman's original illustration oi Athyrium flexile (1854).
and more tapering fronds. When the rhizome is not too deeply embedded amongst
stones the very short stipes are bent over at their bases so that the fronds spread more
or less horizontally, further accentuating the difference in appearance between this
species and the invariably erect A. alpestre. These differences in habit and frond
96
Fig. 86. Athyrium flexile (Newm.) Syme. A pressed specimen collected on Ben Alder
by Dr Sledge in August 1 946. Natural size.
MFC
97
THE OTHER BRITISH YEKNS—POLTSTICHUM, ATHTRIUM, CETERACH
characters enable immature plants of A. alpestre of comparable size to be recognized
without difficulty, and the sterility of such dwarf plants always affords confirmation of
their distinctness. As described by Newman and others, the sori o{ A. flexile are usually
restricted to the basal half of the frond. The sporangia in the sori are always few in
number, not infrequently reduced to three or four or they may even be solitary. In
cultivation the plant remains small and retains all its distinguishing characters.' This is
perhaps demonstrated by Fig. 87.
Owing to the unusually small size of the sori and to the limited number even of these,
it is a matter of some difficulty to obtain dividing mother cells in sufficient abundance
for a satisfactory determination of chromosome number in A. flexile. Out of half a dozen
plants kept in cultivation in Leeds, only one fertile frond was present in the first season
after collection from the wild, and among three similar plants presented to Kew there
Fig. 87. Silhouette of a living frond of Athyrium flexile (Newm.) Syme
after two years in cultivation. Natural size.
was likewise only one fertile frond. Fixings obtained from all available sporangia on
both these fronds only showed some stages of the second meiotic division in sectioned
material, but the second meiotic division is never suitable for determination of chromo-
some number in ferns, and these preparations therefore only serve to demonstrate that
there are the normal sixteen mother cells present which are fairly small and the division
not irregular. With squash preparations success was somewhat better, and about half a
dozen individual cells at the first meiotic division were seen. One of these is shown in
Figs. 82c and 88 and, although this would not in itself be quite conclusive in distinguishing
n = 40 from n — 4.1, the most probable count is n — 40. It may therefore be stated with
confidence that no detectable cytological diflferences appear to exist between the three
British species of Athyrium, and further exploration of the differences between them
would need to be carried out by genetical means.
We may now turn to Asplenium, and here we come to the largest group after Dryopteris
as regards number of species represented in this country, though owing to their small
size (Figs. 89 et seq.) and predilection for rock crevices they are less conspicuous.
There are seven British species. The rarest is Asplenium septentrionale (L.) Hoffm., a plant
of southern affinities only found in a few presumably relict localities in the mountains of
England, Scotland and Wales. A. marinum L. is confined to maritime rocks. A. viride
Huds. occurs both as an alpine and as a lowland plant somewhat resembling Polystichum
Lonchitis in its preference for crevices in limestone rocks, though more widespread and
abundant in individuals than that species. Asplenium lanceolatum Huds. is markedly
Atlantic in its distribution, being found only in scattered localities from Cornwall to
98
THE OTHER BRITISH YERNS—POLTSTICHUM, ATHYRIUM, CETERACH
Cumberland; it is absent from Scotland. The remaining species, A. ruta-muraria L.,
A. Trichomanes L. and A. Adiantum-nigrum L., are fairly generally distributed and often
rich in individuals, the first two at least being familiar occupants of the cracks in masonry
in all our moister districts.
Athyrium flexile n = 40
Fig. 88. Explanatory diagram to Fig. 82 c.
X 1500.
a h
Fig. 89. Two diploid British species of
Asplenium. Natural size. a. A. mari-
«MmL. from a dried wild frond. b.A.
viride Huds.from a living wild frond.
Fig. 90. Meiosis in Asplenium viride
Huds., permanent acetocarmine.
X 1000. ?z = 36.
A . marinum n -36
Fig. 9 1 . Explanatory diagram to
Fig. 93 c. X 2000.
The easiest species to investigate cytologically, though not perhaps the simplest to
collect, are A. viride and A. marinum (Fig. 89). In their chromosomes these two resemble
each other very closely, and both possess a haploid chromosome number of 36 with
no uncertainty. The clearest demonstration of this number is contained in Fig. 90,
which represents a squash preparation oi A. viride, the root of which is represented by
Figs. 93 a and 94. A section of A. marinum is represented in Figs. 91 and 931;.
99
7-2
THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH
Exactly twice this chromosome number, namely, ;z = 72, has been found in all the
other British species of Asplenium, namely, in A. Trichomanes, A. ruta-muraria, A. Adiantum-
nigrum,* A. lanceolatum and A. septentrionale.
Some particularly beautiful squashes showing
the 72 pairs of chromosomes for A. ruta-muraria
and A. lanceolatum are portrayed in Fig. 95a
and b, while a section of ^. Trichomanes mother
cells and a root of A. septentrionale may be seen
in Fig. 93 6^ and b, each beside an appropriate
diploid for comparison.
It is thus clear that the number 36 is as
characteristic and deeply seated in the genus
Asplenium as is the number 41 in Polystichum
and Dryopteris, and if further demonstration
were required it is perhaps appropriate to
mention that these counts have been obtained
not only on British but also, in a number
of species, on continental specimens. In par-
ticular two non-British continental species,
A. fontanum (L.) Bernh. from Switzerland and
A. Petrarchae DC. from the south of France,
have been collected alive as opportunity
offered and grown on for cytological study.
Since both are somewhat unfamiliar to British
readers and one {A. Petrarchae) is also very
rare in its native country, authenticating
silhouettes of the actual living leaves from
which fixations were taken are reproduced in
Fig. 96 fl and b. The chromosome numbers
found in the spore mother cells of these
plants were w = 36 in A. fontanum and n — ja
in A. Petrarchae.
This is all that would now be known
about the cytology of the genus Asplenium but
for evidence supplied by a very well-known
putative species-hybrid found sparingly both
in Britain and on the Continent, to which
the name of A. germanicum auct. non Weiss
(or sometimes A. alternifolium Sm, or A.
Breynii Retz) is commonly given. This plant
Fig. 92. A tetraploid British species oi Asplenium,
A. Adiantiim-nignim L. from Cornwall, from
a living leaf of the plant used, grown in culti-
vation. Natural size.
* Since this was written A. Adiantum-nigrum var. acutum (Bory) PoUini forma lineare Praeger has been
obtained from Madeira and found to be diploid. This may mean that 'J. Adiantum-nigrum^ will need
to be split into two species both of which may actually be in the British flora (cf. Praeger 1934)
although a British specimen of 'var. acutum^ has not yet been examined cytologically.
100
* %
M
}
Fig. 93. Chromosomes of Asplenium in sections, x 1000. a. Mitosis in a root of A. viride Huds. For
explanatory diagram see Fig. 94. b. The same in A. septentrionale (L.) Hoffm. to show higher
chromosome number. For explanatory diagram see Fig. 94. c. Meiosis in A. marinum L. For
explanatory diagram see Fig. 91. d. The same in A. Trichomanes L. to show higher chromosome
number. For further details see Fig. loi.
^»1
^^TV^"**^
,V1»
<?/7= /<? Asplenium ?n ^^ 14-4
A- viride A. sep/^en/-riona/e
Fig. 94. Explanatory diagrams to Fig. 93 a, h. x 2000.
h
Fig. 95. Meiosis in tetraploid species o[ Asplenium, permanent acetocarmine. x 1000. n=i2.
a. A. ruta-muraria L. b. A. lanceolatum Huds.
THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH
(Fig. gja, b) has a very characteristic appearance which does not resemble closely
any other species, least of all those with which it is habitually found. Its hybrid nature
has, however, long been suspected both from its sporadic and sohtary occurrence, single
Fig. 96. Two non-British species of Asplenium, from living leaves, grown in cultivation. Natural size.
a. A. Petrarchae DC. from southern France, b. A.fontamm (L.) Bernh. from Switzerland.
Fig,
a b a
97. Asplenium germanicum auct. non Weiss and its supposed parents, from living fronds grown in
cultivation. Natural size. a. A. germanicum horn V^ ales. Cf. Fig. 98. b. The same from Runmaro,
Sweden, c. A. Trichomanes L. from southern France, a very depauperate specimen, but shown to
be tetraplold. Cf. Fig. 103 a. d. A. septentrionale (L.) Hoffm. from Arthur's Seat, Scotland.
plants only but never populations of similar plants being found, and also from an
apparently invariable association with A. septentrionale (Fig. 97^) and A. Trichomanes
(Fig. 97c). From this circumstance, added to the fact that the spores are always com-
pletely or almost completely abortive, it is generally thought to be a hybrid between
102
THE OTHER BRITISH FERNS— POLTSTICHUM, ATHYRIUM, CETERACH
these two species. Attempts to synthesize it have, however, been curiously unsatis-
factory. Only one author is known to me to have succeeded in carrying out a cross
between A. septentrionale and another species of Asplenium, namely Heilbronn (1910),
but even he was forced to point out a dissimilarity between the hybrid he produced and
the natural one.* It is true that a surprisingly contradictory statement was made by a
distinguished amateur, P. Kestner, who remarks in the British Fern Gazette of 1935 that
A. germanicum is the only fern hybrid that can be synthesized with ease. This statement,
Fig. 98. The Welsh plant of Asplenium germanicum auct. non Weiss in its original habitat. Natural size.
From a photograph kindly supplied by the finder, Dr H. F. Dovaston.
though perhaps true as will be seen below, cannot unfortunately be used as evidence,
since Kestner failed to authenticate it by keeping a record, photographic or otherwise,
of the plants he refers to.
I am fortunate in having had access to living specimens of A. germanicum from several
different countries. A British specimen from south Wales, a leaf of which is shown in
Fig. 97a, was made available to me by the finder, Dr Dovaston, whose photograph of
it in its original habitat is also reproduced (Fig. 98). Two continental specimens were
* Heilbronn claims to have crossed A. septentrionale with A. ruta-muraria. This should have yielded
A. Murbeckii Dorfl. and it is therefore not quite clear why the author was looking for A. germanicum,
although his photograph does show some points of resemblance to A. germanicum.
103
THE OTHER BRITISH FERNS— POLTSTICHUM, ATHYRIUM, CETERACH
found by me on separate occasions in the summer of 1937 in the North Itahan Alps
and brought aHve to England, where they unfortunately perished during the war, but
not before successful fixations had been made. To replace them additional material
O
0
o
<^«^%-^0
d?0
o
0
O ^ Oof)
(p
/4. germanicum 3n = /^5
Fig. 100.
Fig- 99-
Fig. 99. The cytology of /ii-/)/fm«m ^ermaw/cM?n auct.non Weiss, x 1000. For explanatory diagrams see
Fig. 100. a. Meiosis in a section of an Italian specimen showing lagging univalents, b. Meiosis
in the Welsh specimen in balsam after acetocarmine showing pairs and univalents, c. Two focal
levels through a mitotic figure in a root of the Welsh plant showing the triploid chromosome
number.
Fig. 100. Explanatory diagrams to Fig. 99^*, c. x 2000.
was obtained in the summer of 1948 consisting of one plant from the island of Runmaro
near Stockholm, Sweden, kindly procured for me by Professor Halle and posted to the
Royal Botanic Gardens, Kew; and a Swiss plant found near Sion by my colleague. Miss
Davies, and brought back by her to Leeds. A leaf of the Swedish specimen is reproduced
in Fig. 97^.
104
THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH
All these plants agree very closely in their cytology. They show unmistakable signs
of hybridity in the irregular appearance of the first meiotic division, which may perhaps
be sufficiently demonstrated by Fig. 99 a and b. The first of these shows a section of one
of the Italian specimens with numerous unpaired chromosomes; the second (Fig. 99^)
is a single mother cell in an acetocarmine preparation of the Welsh specimen in which
fuller details of pairs and univalents can be made out.
In all cases, however, the chromosome number was anomalous. The first detailed
count had been made on a root of one of the Italian plants in which about 100, and not
the expected 144, chromosomes had been found. This seemed at the time so inexplicable
that the record was at first discarded as a possible case of mis-identity since the plant
had died during the war without a herbarium record of it having been preserved.
The same result, however, was obtained with the British plant, as may be seen from the
photographs and diagrams of Figs. 99 and 100. When, as a result of this discovery,
the two additional continental specimens (from Sweden and Switzerland) had been
examined and an exactly similar result obtained in each, the facts could no longer be
doubted; A. germanicum in four European countries and therefore probably always, is
not a tetraploid as its putative parentage would suggest but a triploid (3^ = 108).
The discovery of a triploid hybrid where a tetraploid was expected has already
occurred in the case o{ Dryopteris remota (see Chapter 5), but since Asplenium germanicum
differs from Dryopteris remota in not being apogamous, the possible explanations of its
origin are less numerous. Since the plants are sterile they cannot have been dissemi-
nated by stray spores from a common source, and it must be assumed that each has
arisen de novo wherever it is found. Since they are all very similar morphologically a
complex origin by a, process of segregation from a previous hybrid of different chromo-
some number seems impossible. It remains, therefore, to be seen whether a diploid
form of one or other of the parent species can be found from which a triploid could be
produced directly as a simple hybrid.
In searching for this it was felt at once that Asplenium Trichomanes was the more prob-
able species to be involved, since A. septentrionale had already been examined cytologic-
ally several times from A. germanicum localities; moreover, it is so uncommon in Great
Britain that the probability of finding more than one form of it in this country at any
rate seemed remote. With A. Trichomanes, on the other hand, the position is different.
It is so abundant and familiar that it had not previously been felt necessary to take
special precautions to collect it from the immediate vicinity of the hybrid, and reliance
had been placed on the wealth of wild specimens nearer home. This meant that while
chromosome counts, invariably tetraploid, had been made on specimens of /I. Tricho-
manes from Devon, Derbyshire, Yorkshire and the south of France, none of these plants
had actually been associated with either A. septentrionale or A. germanicum, and it was
conceivable that had such an association been insisted upon a different result might
have been obtained. Attention was therefore directed towards sampling A. Trichomanes
populations from these precise habitats.
At once what was looked for was found. A form of A. Trichomanes obtained from
Snowdon (Wales), and transferred to Kew in 1946 was found to be diploid. The two
chromosome numbers are demonstrated in Figs. 1 01-102, and a leaf of each type
105
THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH
is shown in Fig. 103. The most obvious morphological difference is the crenate
edge of the diploid, but it is still too soon to say exactly what diagnostic criteria will
best serve to distinguish it in the field. It is also at present uncertain whether the
tetraploid in this case is an autopolyploid or an allopolyploid. The most that can be
said is that quadrivalents are not obvious and seem to be absent; the very close morpho-
logical similarity of the two forms raises the question, however, which must also be
answered for the comparable cases of Dryopteris dilatata and D. Villarsii, as to whether
Fig. loi. The two forms of Asplenium Trichomanes L. in Britain, permanent acetocarmine. x 1000.
For explanatory diagrams see Fig. 102. a. Tetraploid plant from Ashburton (Devon) with leaves
as in Fig. 103a. b. Diploid plant from Snowdon (Wales). Cf. Fig. 1036.
/J. /r/chom^nes n^ 72 n = 36
Fig. 102. Explanatory diagrams to Fig. loi. x 2000.
an autopolyploid can perhaps lose its power of multivalent formation after a sufficient
lapse of time. Only further experiment can elucidate this matter, but in the meantime
the results so far make it extremely probable that Asplenium germanicum is indeed a
species-hybrid as originally postulated but that the diploid form of ^. Trichomanes and
not the tetraploid must be used. With this information it should only be a matter of
time before this hitherto perplexing hybrid has been synthesized.
So close to Asplenium that it must certainly be discussed with it is Ceterach, the 'Rusty
Back' (Fig. 104). This small genus of only 3-5 species in all has just one representative
in Europe, C officinarum Lam. & DC. This is locally abundant in many parts of Great
Britain, especially in the west, and it shows a marked preference for somewhat calcareous
106
Fig. 104. Ceterach officinarum Lam. & DC. a fresh frond of a
British specimen. Natural size.
Fig. 105. Meiosis in Ceterach officinarum Lam. & DC,
permanent acetocarmine. x 1000.
^*
Ceterach n -- 72
Fig. 106. Explanatory' diagram to Fig. 105. x 1500.
Fig. 103. Living leaves of the
two forms of Asplenium Tri-
chomanes.
a. Tetraploid, cf. Fig. loia.
b. Diploid, cf. Fig. loi ^. Kew
reference no. 157.46.
THE OTHER BRITISH FERNS— POL 15 T/C/Zt/M, ATHTRIUM, CETERACH
habitats. This preference is even more marked on the Continent, where Ceterach shows
its remarkable power of withstanding seasonal drying up by being one of the most
characteristic ferns in the cracks of glaring white limestone on the north Mediterranean
coast. Taxonomically it differs from Asplenium chiefly by the profusion of papery scales
on the backs of the fronds ; apart from this and the somewhat fugaceous indusium, the
soral characters of the two genera are very similar.
That Ceterach is closely related to Asplenium is further indicated by the cytology. In
both English and French specimens I have found 2« = 144 and /z = 72,* as in the majority
o{ Aspleniums. One photograph (Fig. 105) and a diagram (Fig. 106) are appended in
illustration.
The cytological confirmation of the suspected close affinity between Ceterach and
Asplenium brings into strong rehef the contrasting case o{ Athyrium, and we may therefore
fittingly close this chapter by drawing attention to a not unimportant conclusion which
both Athyrium and Dryopteris have brought out. This is the unreliability of soral char-
acters when taken alone as a guide to phylogeny. In the previous chapter we were
forced to accept the conclusion that the apparent resemblance of Dryopteris in the
narrow sense to Thelypteris was due to parallel evolution from different ancestral stocks,
and we are now confronted with the same situation in Asplenium and Athyrium. The sum
of anatomical characters, and chromosome number, are both more reliable than are the
details of the sorus when taken alone, as an index of affinity. This fact is perhaps of
some importance for taxonomists to know.
SUMMARY
As in the previous chapter, the most suitable factual summary is perhaps merely a list
of the chromosome numbers recorded in it, and this is appended. As points of special
interest attention may perhaps be directed to the new observations on the very rare and
little known Athyrium flexile (pp. 95-98), to the unexpected facts regarding the hybrid
Asplenium germanicum (pp. i oo-i 06) , and to the existence of a diploid as well as a tetraploid
form of Asplenium Trichomanes. The diploid form of the last species has only recently
been discovered and requires further study. It is present in Britain and probably on
the Continent, but in Britain at least is less common than the tetraploid.
* This number has since been found also in Ceterach aureum (Cav.) v. Buch from Teneriffe.
108
THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH
List of chromosome numbers recorded in this chapter
Name
Source
Roots
Meiosis
Remarks
Polystkhum:
P. Lonchitis (L.) Roth
Ireland
• 82
41
Diploid species
Scotland
.
41
>> j>
Switzerland
41
>> >>
P. setiferum (Forsk.) Woynar
England
.
41
>> >'
{P. angulare Presl)
Switzerland
82
41
5> J>
P. aculeatum (L.) Roth (P. lo-
England
164
82
Tetraploid species
batum (Huds.) Woynar)
Switzerland
164
82
>j »
Athyrium:
A. Filix-femina (L.) Roth
England
•
40
Diploid species
Scotland
•
40
>) >>
A. alpestre (Hoppe) Ry lands
)»
80
.
>> >>
A. flexile (NewTiian) Syme
»j
80
40
>> >>
Asplenium:
A.fontanum (L.) Bernh.
Switzerland
72
36
>> >>
A. viride Huds.
England
72
36
>> >>
A. marinum L.
England (Devon)
•
36
5> JJ
Ireland
«
36
>» >>
A. Adiantum-nigrum var. acutum
Madeira
72
•
» (?)
(Bory) Pollini
A. ruta-muraria L.
Devon
•
72
Tetraploid species
A. lanceolatum Huds.
»>
.
72
» >>
A. Adiantum-nigrum L.
9)
.
72
>> >>
A. septentrionale (L.) Hoffm.
Scotland (Arthur's Seat)
.
72
» j»
Switzerland
144
.
jj j>
A. Petrarchae DC.
Southern France
.
72
>> >>
A. Trichomanes L.
Devon \ <
Yorkshire
144
72
)) 5J
Southern France
Perhaps auto
Wales
72
36
Diploid species (?)
A. germanicum auct. non Weiss
Sweden '
Italy
c. 100
(probably 108)
Irregular
Triploid hybrid
Wales
Ceterach:
C. officinarum Lam. & DC. Devon
Southern France
C. aureum (Cav.) v. Buch Teneriffe
72
72
72
Tetraploid species
109
CHAPTER 7
THE OTHER BRITISH FERNS (CONTINUED)
Two genera which should perhaps be considered with the Dryopteroids are Woodsia
and Cystopteris, but both are somewhat more doubtful in position than those dealt with
in the previous chapter, and for this reason discussion of them was deferred. As will be
seen by reference back to the diagram on p. 89, the phyletic affinities of both are
marked as questionable by Bower. With regard
to Woodsia the general consensus of opinion seems
to be that it is probably a primitive genus related
to Dryopteris, either directly by way of certain tree
ferns known as the Cyatheaceae (Bower's view)
or indirectly (view of Christensen, Copeland,
etc.). Cystopteris, according to Bower, is frankly
'incertae sedis', but other authors (e.g. Newman,
1 854) place it with confidence near Woodsia.
Taking Woodsia first, this is represented in
Europe by three species, only two of which are
British. W. ilvensis (L.) R.Br, and W. alpina (Bol-
ton) S. F. Gray [W. hyperborea R.Br.) are among
our rarest ferns, being confined to a few localities
in the mountains of Scotland and Wales, in most
of which their numbers, unhappily, appear to be
diminishing. W. ilvensis, originally the commoner
and somewhat the larger of the two, is now quite
extinct in many of its classic haunts, though
fortunately it still lingers on a few unfrequented
and inaccessible crags. For precise instructions
as to how to reach one of these I am greatly
indebted to my friend, Dr H. F. Dovaston. Thanks to this, and with the help of Professor
D. Thoday of Bangor, I was able in July 1945 to collect a living fertile frond from an
authentic wild Welsh plant and to bring one small offset, from a group proliferating
from the base of a dead stock, into cultivation. Both the frond and the offset yielded
cytological information. For the other species I am indebted to Mr G. Roger of Manchester
Museum and to Dr B. T. Cromwell of Hull, each of whom has lent me one living plant
collected wild in Scotland. Silhouettes of fronds of the plants examined are given in
Fig. 107a and b.
The chromosomes of the two species are shown in Fig. 108 a and b (explanatory dia-
grams in Fig. 109 a and b). It is obvious at a glance that they are not identical and that
one species {W. alpina. Fig. 108^) has about twice as many chromosomes as the other
{W. ilvensis, Fig. io8fl). As nearly as can be determined the exact numbers are n = 41
Fig. 107. British H'ooi^^fa from living leaves
grown in cultivation. Natural size. a.
W. ilvensis (L.) R.Br, from Wales, b. W.
alpina (Bolton) S. F. Gray from Scotland.
I 10
THE OTHER BRITISH FERNS
for W. ilvensis and « = 82 for IV. alpina, and the maximum possible error is in both
species no more than one chromosome in the monoploid count. This uncertainty arises
from the presence of what appears to be a foreign body accidentally superimposed on
the cell of Fig. 108 a and indicated as such in the diagram. It gives, however, a super-
a b
Fig. 108. Meiosis in British pyoorfyfa, from permanent acetocarmine preparations, x 1000. For explana-
tory diagrams see Fig. 109. a. W. ilvensis (L.) R.Br. ^ = 41. b. W. alpina (Bolton) S. F. Gray.
n = 82.
Woods la ilvensis n= 41 Woodsia alpina n - 82 ^
a b
Fig. 109. Explanatory diagrams to Fig. 108. x 1500.
ficial resemblance to an extra chromosome. As long as slight uncertainty affects the
monoploid count it would be unwise to depend on the higher number of the other species
(Fig. lo^b) for greater precision. These numbers should therefore for the moment be
accepted as probable, though not quite certain. The fact that in Woodsia we must for
the moment be content with less than the usual precision is due to the very delicate
1 1 1
THE OTHER BRITISH FERNS
nature and small size of these plants, coupled with their extreme rarity in Britain,
which makes a large-scale multiplication of preparations unusually difficult. Other
cells of both species have, of course, been seen, but none of better quality.
Turning now to Cystopteris we come to what is in some ways the most remarkable
cytological problem yet raised in the ferns. C.fragilis (L.) Bernh. is a small rock plant of
world-wide distribution, being met with even in the southern hemisphere. It is a vari-
able plant, and it has often puzzled systematists to decide whether it should or should
not be subdivided into several species. A closely cognate problem is to decide whether
C. alpina Desv. (C. regia Desv.) from the central European mountains should be separated
from it as a species, and, if so, whether this species is present in the British Isles. Com-
parable difficulties concern the status of other extreme foliar types, C. Dickieana Sim,
once found near Aberdeen, being one of the most frequently debated. The difficulty
1^1^^^
Fig. iio. Meiosis in Cystopteris fragilis (L.) Bernh. in section, x looo.
The chromosomes uncountable, but see Figs. 1 12-1 15.
with all these plants is their extreme plasticity under different environmental conditions,
so that without experimental cultures it may be quite impossible to obtain sufficient
evidence from herbarium material alone, and even with experimental cultures it may
require far more difficult types of observation than mere comparison of foliage.
In contrast to the host of problems raised by or round C.fragilis (L.) Bernh., the other
British species, C. montana (Lam.) Desv., is simplicity itself With its creeping rhizome and
deltoid leaves it is as distinct from the fragilis complex as is the Oak Fern from the Lastrea
type of Dryopteris. It is, however, excessively rare in the British Isles, being confined to
central Scotland. Although I have had a specimen from this region in cultivation,
I have not so far been able to obtain a cytological result from it. In the account
which follows I have had to be content with a specimen of garden origin, previously
collected in Switzerland.
Before presenting what can only be regarded as a prehminary report on the cytology
of Cystopteris, a special note on technique is perhaps desirable. In my experience this
genus is more than usually difficult to study effectively by any of the older cytological
methods. In sections of roots the hundreds of thin and contorted chromosomes are
virtually uncountable, and the same is true of meiosis, as may perhaps be seen from Fig.
1 12
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"3
THE OTHER BRITISH FERNS
no, in which the chromosomes at metaphase are present in layers. Only by the
squash method, details of which are explained in Appendix i, can success be
achieved, and although this has been used with effect in the work described
in several previous chapters the precise details of its application to the higher ferns
were in the first instance devised to meet a complete technical deadlock which had
descended on the genus with which we are now concerned. The introduction of the
squash method resolved the technical impasse at once, but thereby revealed a cyto-
t
■
li
r
S^"^ « _
•
• ^
♦
•
k
•
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-
Fig. 112. Meiosis in 'Cj'j/o/^/fm aZ/ima' from the leaf of Fig. 1 1 1 i. Fresh acetocarmine. x looo. n= 126.
For explanatory diagram see Fig. 113.
genetic problem of quite unusual complexity, a circumstance which was nevertheless to
be expected in view of the well-known taxonomic confusion prevailing in the genus.
The first squash preparation to give a satisfactory result was that of Figs. 112 and 113.
The leaf from which it was obtained is shown in Fig. \i\h, and its very finely cut pinna-
tion characteristic of 'C. alpina'' is clearly seen. This specimen, which was kindly given
to me by Dr Rowlands of Doncaster, was described by him as 'the most authentic form
of C alpina (C. regia) to be found in Switzerland', since it retains fully its distinctive
characters in cultivation.* This is by no means always the case with alpina-like forms
*The origin of this particular material is discussed in several letters included in volume vi of the
British Fern Gazette (1931). The finely cut specimen of C. regia collected by Waltham is said to have come
from 'limestone above Geneva at about 4600 ft.' in 1926 (loc. cit. 1931, p. 37). My plant agrees exactly
with that figured on pp. 72 and 85 of this volume of the Fern Gazette.
1 14
r • • M.
*• ' •
C. "dipina " n - /26
Fig. 113. Explanatory diagram to Fig. 112. x 1000.
^V^
A
9^
ii
r«^
♦
X?
4t
4
>^
d'
Ci/stopteris Dickieana n = 84
Fig. 114. Meiosis in Cystopteris Dickieana Sim, fresh Fig. 115. Explanatory diagram to Fig. 114.
acetocarmine. x 1000. From the leaf of Fig. 117a. x 1000.
For explanatory diagram see Fig. 115.
115 8-2
THE OTHER BRITISH FERNS
from high akitudes which often tend to revert to the more usual form ofC.fragilis when
grown at sea-level. The chromosome number of this specimen is not in doubt. As
comparison between the photograph and the explanatory diagram should prove there
are exactly 126 pairs of chromosomes in this cell.
A gametic chromosome number of 126 was subsequently found many times in plants
of very diverse origin and appearance. A small array of relevant leaves is contained in
DICKIE'S FERN, {natural size).
Fig. 1 16. Newman's original figure of Cjstopteris Dickieana Sim
(after Newman, 1854). Natural size.
Fig. Ill, plants of both Swiss and British origin being represented, and two further
samples of cytological preparations together with one additional explanatory diagram
are placed later in the chapter in Figs. 120 a and b and 121b. In spite of the considerable
range of morphology there appears to be no detectable cytological difference between
^C. fragilis' types and 'C. alpina' or alpina-\ike types. The reality or otherwise of
C. alpina as a justifiable species can therefore only be further investigated by genetical
means.
An extended search through European populations of Cystopteris has, however, shown
that some genuine cytological diflferences do exist within the C.fragilis complex, some of
116
THE OTHER BRITISH FERNS
which may perhaps be of taxonomic importance. The first preparation, to give a chromo-
some number different from 126 was that of Figs. 1 14 and 115 which refer to C. Dickieana
Sim, a leaf of which is shown in Fig. 117a. There are 84 chromosomes only.
C. Dickieana is a well-known horticultural 'variety' which was originally found wild
on the coast near Aberdeen and subsequently in a very few other parts of Scotland
Fig. 1 1 7. Forms oi Cystopteris with verrucose spores from living leaves grown in cultivation. Natural size.
a. C. Dickieana Sim. b. Specimens from Greenland (see text). c. 'C BaenitzU' from the type
locality in Norway.
(Druce, 19 19) though probably now exterminated there except in cultivation.* It was
described by Newman in 1854 as a probable species, and it has retained all its dis-
tinguishing marks with great constancy in cultivation, as comparison of Newman's
figure (Fig. 116) with my specimen (Fig. iiyfl) will perhaps indicate. Newman's
view that this might be a new species was not wholly based on the leaf morphology, in
* Notes on the origin of C Dickieana will be found in vol. vi of the British Fern Gazette, notably pp. 18
and 19 (Rowlands, 1929). The original description will be found in Newman (1854).
117
THE OTHER BRITISH FERNS
which the congested pinnae are the most conspicuous feature, but was also based on the
observation that the spores have a highly characteristic surface pattern when seen under
the low power of the microscope. This pattern has been described as 'verrucose'
(Fig. 1 1 8c) to distinguish it from the very spiny surface met with in other forms of
C.fragilis (Fig. ii^a,b) and C. alpina.
The first reaction to finding a distinctive spore pattern as well as a new chromosome
number in C. Dickieana is perhaps to strengthen Newman's conclusion that here is a
new species. This is probably the correct interpretation, but before redefining the
specific characters it is desirable to make comparisons with material from other sources
if possible, and unfortunately the moment this is attempted uncertainty of a different
sort sets in. . ^
The fronds reproduced on Fig. 1 1 7 have all spineless spores and the low chromosome
number (;z = 84). In leaf morphology they are, however, very diverse. The leaf of
Fig. 118. Spore forms in C>i^/o/^fem. x 250. a. The large spiny spores associated with the high chromo-
some number (n= 126) from the leaf of Fig. 1 1 1 Z>. b. Slightly smaller spiny spores associated with
the lower chromosome number (« = 84) from the leaf of Fig. 119c. c. C. Dickieana Sim with
verrucose spores.
Fig. 117c is from Kongsvold, Dovrefjell, Norway, from a plant kindly sent to me alive
in 1948 by Mrs Gunvor Knaben of Oslo. Kongsvold is the type locality for ' C. Baenitzii',
which was described in 1891 by Dorfler, and named after its discoverer, one Baenitz
(1891), as a species, on the sole criterion of spineless spores. The leaf of Fig. 117c can
hardly be other than C. Baenitzii, and it is not more different from C. Dickieana than is, for
example, C. alpina from C.fragilis, though it lacks the congested pinnae of C Dickieana
from Scotland. The leaves of Fig. 117^ are, however, of an extremely different type.
They are from a plant brought back alive from Greenland in the autumn of 1948 by the
Leeds University Expedition to that country. They represent an extreme arctic type,
having been laid down in the bud in the original locality which was within five miles
of the edge of the permanent ice-sheet which covers central Greenland. Immediately on
arrival in Leeds the leaves expanded and within three weeks had given both a cytological
demonstration of n = 84 and also proof of spineless spores. To what extent their appear-
ance will alter in subsequent years after continuous growth under more temperate
conditions cannot be predicted.
Preliminary search for spineless spores in herbaria has shown them to be distributed
on a world scale, although in western Europe they are very infrequent. To the one
locality in Scotland (that of var. Dickieana itself) can be added several records of
118
n^-ry
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"9
THE OTHER BRITISH FERNS
C. Baenitzii Dorfl. in Norway, north Sweden and Finland and quotations of records in
the flora of Russia (Komarov, 1934) from right across Siberia. From the south there
are specimens in the Kew herbarium from Algeria, Asia Minor, Persia and the
Himalayas. They also seem to be present in North America.
r-
mif'^. "am ^Mrv
*«^vfC7
1'
<■'*•
L
% f
4
3i
t*
»« ^
4» W • -
^' »' V *
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%-it'^'^r'*-^ f^
^ ■,
4 *• '^ J
c d
Fig. 120. Meiosis in various QvJ'to/^/mj types, acetocarmine. x 1000. a. 'C./ra^//w' from Ingleborough,
Yorkshire. «=I26. b. 'C. alpina forma obtiisa Kestner' from Switzerland, from the leaf of
Fig. Ilia, n = 1 26. For explanatory diagram see Fig. 1 2 1 6. c. ' C fragilis ' from Canada from the
leaf of Fig. 119c. 72 = 84. For explanatory diagram see Fig. 121a. d. C. montana (Lam.) Desv.
« = 84. For explanatory diagram see Fig. 122.
With such a vast range to explore a precipitate definition of species would be unwise.
In terms of populations, however, we seem to be dealing with an ancient and perhaps
relict stock of arctic affinities which is probably not co-specific with the spiny-spored
Cystopteris, though whether it represents one species or several cannot yet be known.
120
THE OTHER BRITISH FERNS
It is also desirable to look for genetical evidence regarding the mode of inheritance of
spore pattern, before final conclusions are drawn.
The status of the C. Dickieana-Baenitzn complex would be easier to determine if there
were greater uniformity among the spiny-spored forms. These are, however, more
diverse than the description so far given suggests. It is true that in Britain and Switzer-
land the principal populations have both spiny spores and the high chromosome number
of 126, but even in these countries other types can be found, and elsewhere the relative
proportions may prove to be quite different. Fig. 119 shows a small assemblage of
V
^\
^T \^^
n -- 126
Cystopteris n ^ 84
a b
Fig. 121. Explanatory diagrams to Fig. 120c and b to show the two chromosome numbers
in ^ Cystopteris fragilis'' with spiny spores, x 1500.
fronds from Britain, Switzerland, Scandinavia and Canada in which rather small
spiny spores and the lower chromosome number of 84 have been found. Populations
of this type have been met with so far in one place in Switzerland (Preda), in two places
in Britain (Rannoch Moor in Scotland and the Lake District), in Iceland (Brekkufjall),
in Finland (Piikkio), and over large areas in Scandinavia, specific sites being Runmaro
near Stockholm, Storlien in Jamtland, Swedish Lapland, and Trondheim and Hell
in Norway. It is probable, indeed almost certain, that the high-numbered form also
occurs in Scandinavia, though it is not so distinctly the dominant type there as it is
with us. In Canada, on the other hand, the high-numbered form has not yet been
encountered, although the low-numbered form has been obtained from Ontario,
Ottawa and several places on Vancouver Island (Figs. 120c, 121a).
Taking the whole evidence assembled to date we have thus detected in a preliminary
glance at Cystopteris in Europe, Iceland, Greenland and America three spore types
(i.e. large and small spiny spores and verrucose spores), two chromosome numbers
121
THE OTHER BRITISH FERNS
(n = 126 and 84) and a great variety of different forms of leaves. We may be certain
of one thing only, which is that this degree of diversity cannot be all that will be found.
For one thing the cytology as it stands is manifestly incomplete. The relationship
between numbers as different as 126 and 84 may not at first leap to the eye, yet we
can hardly doubt that we are dealing with the upper members of a polyploid series,
the lower ones of which are still to seek. If a form with a gametic number of 42 could
be found we should have a simple series of 42, 84 and
1 26 in which the plants bearing them would be diploid,
tetraploid and hexaploid respectively. It may be that
the diploid is extinct. In a world-wide range it is,
however, scarcely necessary to postulate extinction ^
as long as we are in ignorance of the nature of popula-
tions over vast regions. It is, therefore, desirable to '^ _ .^
continue the search for cytogenetic data on a scale ^H^i^L^'T
far greater than has so far been obtained, in the light ^ ^ 4 ^*
of which the speciation of this most troublesome ^f ^^' ^
group may perhaps become clearer. - " ^m £a
In the meantime perhaps enough has been said to / | ^^^ ^
give some indication as to why the pure taxonomy of
the Cystopteris populations of Europe is so intractably
entangled when studied without cytology, and to c. montana n - 64
hope that more extensive collections, especially from Fig. 122. Explanatory diagram to
countries outside Britain, may ultimately bring order Fig. 120^. x 1500.
out of chaos.
The other British species, C. montana (Lam.) Desv., can be dealt with in one sentence.
The chromosome number (Fig. 122) is 84, a fact which is of importance merely in show-
ing that multiples of 42 are no innovation in the C. fragilis complex, but are probably
fundamental to the genus. In this respect therefore the genus Cystopteris, though
perhaps related to the Dryopteroids and to Woodsia, nevertheless stands somewhat alone.
Leaving the Dryopteroids now aside, the next major group to be placed near them by
Bower (see diagram, p. 89) is that of the Blechnoids. The British representatives com-
prise two genera, Blechnum and Scolopendrium* [Phyllitis), each with one species only in
this country, namely Blechnum spicant (L.) With, and Scolopendrium vulgare Sm. ( = Phyllitis
Scolopendrium (L.) Newman). Both genera are commonly regarded as in some way
related to Asplenium, though the interpretation of the nature of the relationship varies
with different authors ; Bower himself regards Scolopendrium as the end-result of a series
of developments in the order Asplenium — Blechnum — Scolopendrium, but the alternate
order, namely, Asplenium — Scolopendrium — Blechnum, is that more commonly adopted in
Floras (cf also Copeland, 1947).
The chromosome numbers of the British species are shown in the photographs (Fig.
123^,/). Blechnum spicant (Fig. 131/) has n = 34, the number having been established
*The retention oi Scolopendrium instead of the technically more correct generic name Phyllitis for the
purpose of this chapter is a matter of convenience in equating it with the bulk of the literature dealing
with this species.
122
Fig. 123- Meiosis in various genera. Acetocarmine. x looo. a. Polj'podiumvulgarevar.semilacerumllort.,
permanent preparation. n = 37. b. Adiantum capillus-veneris h., permanent. n = 30. For explanatory
diagram see Fig. 125. c. Cryptogramma crispa (L.) R.Br., fresh preparation. n=:6o. d. Pteridium
aquilinum (L.) Kuhn, permanent preparation. « = 52. e. Scolopendrium vulgare (L.) Newm., fresh
preparation. ^ = 36. For explanatory diagram see Fig. 124. /. Blechnum spicant (L.) Roth,
permanent preparation. « = 34.
123
THE OTHER BRITISH FERNS
for specimens from both Scotland and England. Scolopendrium vulgar e^ on the other hand
(Figs. 123^, 124), has n = 36. This result has been obtained for plants from both the
south and the north of England and in some horticultural strains (cf. Chapter 11);
there seems, therefore, to be no doubt as to its accuracy.
In so far as generic chromosome numbers based on single species mean anything,
these numbers, while supporting the general resemblance of both genera to Asplenium
(for which n = 36), also give some slight emphasis against Bower's interpretation of the
form of the relationship and in favour of the older view that Scolopendrium is closer to
Asplenium than is Blechnum. This view, it may be said in passing, is also that adopted by
Holttum (1947).
Some additional information * about the genus Scolopendrium will be found in Chapters
8 and 1 1 ,
#V »
Scolopendrium n-36 Adiantum n^SO
Fig. 124. Explanatory diagram Fig. 125. Explanatoiy diagram
to Fig. i23e. X 1500. to Fig. 1236. x 1500.
All the remaining genera of British ferns are, like the last two, either monotypic in
this country or at least appear to be so at first sight. With one exception (see next
chapter) they need not detain us long.
Of the Pteroid affinity (see diagram, p. 89) we have only the Common Bracken,
Pteridium aquilinum (L.) Kuhn. This species is one of the most widespread plants known
* The establishment of « = 36 for Scolopendrium vulgare is also of interest in another connexion. This
species is one of the relatively few ferns for which a considerable body of genetical information exists.
It was the first member of the group for which simple Mendelian inheritance was demonstrated (Lang,
1923) and has since been the object of closer study by Andersson-Kotto. Of special interest in the latter
work was the strain known as 'peculiar' for which the peculiar characteristic was the tendency of the
edges of all the leaves to become prothalloid without special treatment to bring this about. As in other
cases of apospory (cf. Osmunda) such prothalli could become free living if laid on soil and could produce
new sporophytes from apparently normal sex organs. A sequence of such plants should give rise to a
polyploid series, as in Osmunda. In Scolopendrium, however, an anomalous relation of chromosome
numbers was described (Andersson-Kotto and Gairdner, 1938), but it is now evident that their analysis
of 'peculiar' Scolopendrium is somewhat vitiated by the fact that their initial estimate of the chromosome
number for the parent species was seriously in error. These authors had assumed that their material
must have started with « = 30 and 2« = 60. With the present demonstration that the correct numbers are
n = 36 and 2n — 72 it is clear that the cytology of 'peculiar ' Scolopendrium needs to be reinvestigated before
any interpretation is possible.
124
THE OTHER BRITISH FERNS
to science, for, without being a weed of cultivation, it is nevertheless to be found in
every continent in the world. I am fortunate in having been able to compare a tropical
and a temperate individual, fixed material of a specimen from Malay having been sent
to me in 1938 by my friend Dr Chapman. The Malay material proved indistinguishable
at meiosis from a British plant from Lancashire, both having n = 52 (Fig. 12'^d).
Of the Gymnogrammoid affinity we have the Parsley Fern Cryptogramma crispa (L.)
R.Br. { = Allosorus crispus (L.) Bernh.), and the Maiden Hair, Adiantum capillus-veneris L.
The little annual Anogramme leptophylla (L.) Link, though present on the island of Jersey,
may perhaps be omitted from the present survey since it does not touch the mainland
of Great Britain. Owing to the war it has in any case been impossible to obtain it.
Taking the Maiden Hair [Adiantum) first, this is a rare British plant though exceed-
ingly common in many of the warmer parts of Europe. I have examined it cytologically
in wild specimens from Italy and from Spain and in one British example brought back
from the limestone pavement of Galway Bay in Ireland. All three specimens gave
approximate root-tip counts of 2n = c. 60, and they seemed indistinguishable. The
Irish specimen unfortunately perished in an air raid before meiosis had been examined.
Both the continental specimens, however, give n = 30 without any ambiguity (Figs.
123^, 125). This may therefore be accepted for the British plant also.
The Parsley Fern Cryptogramma (or Allosorus) is exceedingly abundant as a scree
plant on siliceous rocks in many of our mountain regions, such as the Lake District and
Wales. I have investigated it from the Lake District and, as shown in Fig. 123c,
n = 60. The resemblance between this and the preceding is clearly of the sort which
may be expected to be of use for taxonomic purposes when a greater number of species
of the Gymnogrammoid affinity have been studied.
Reviewing this chapter we may note the rather wide range of different chromosome
numbers to be met with in Britain as soon as we leave the relative uniformity of the
Dryopteroid affinity behind. This may perhaps suggest that in the Pteridophyta as in
the Cruciferae the aneuploid numerical changes, though rare, are actually concerned
with the formation of larger evolutionary units than those which result with almost
monotonous frequency from polyploidy. This conclusion is of some importance and
should if possible be pursued outside the rather narrow confines of the European flora.
125
THE OTHER BRITISH FERNS
SUMMARY
Summarizing the facts of the chapter we may quote the following list;
Species
Source
Woodsia:
W. ilvensis (L.) R.Br.
Wales
Probably 41 (41-2)
IV. alpina (Bolton) S. F. Gray
Scotland
Probably 82 (82-4)
Cystopteris:
C. montana (Lam.) Desv.
Hort. (Switzerland)
84
C. Dickieana Sim
Hort. (Scotland)
84
'C. Baenitzii Dorfl.'
Norway
84
Cystopteris, smooth spores
Greenland
84
C.fragilis (L.) Bernh., spiny spores
Norway, central and north Sweden, Runmaro
(near Stockholm)
84
Iceland, Finland
84
Rannoch (Scotland), Lake District
84
Switzerland (one locality)
84
East and west Canada
84
C. fragilis
Britain, many localities
126
Switzerland, many localities
126
South Sweden
126
'C. alpina' Desv.
Switzerland
126
Blechrmm:
B. spicant (L.) With.
Britain
34
Scolopendrium:
S. vulgare Sm.
Britain
36
Pteridium:
P. aquilinum (L.) Kuhn
Britain and Malay
52
Adiantum:
A. capillus-veneris L.
Britain, Italy, Spain
30
Cryptogramma:
C. crispa (L.) R.Br.
Britain
60
126
CHAPTER 8
POLTPODIUM VULGARE
In the first draft of this manuscript one paragraph at the end of Chapter 7 was to have
been devoted to the last of the higher British ferns, Polypodium, and the inclusion of a
photograph of the chromosomes of 'Polypodium vulgare var. semilacerum' on Fig. 1230,
p. 123, is a reminder of that intention. As the work of writing has progressed, however,
£a,cccljr]r.
jpiipodium latine.gtcccoiptm's
'Brabiccbif bcygvd biftc vcl fill
cic3;^a3pion li.aggtccapro blf
igaucto:it3tcjJ»faf.3a)ifbci'g.i.-^oIi
Fig. 126. Woodcut illustration of Polypodium in the Ortus Sanitatis of 1491. From a copy in the Rylands
Library, Manchester. Slightly reduced. Portions of the Latin text are visible below the drawing.
the unexpected complexity of the Polypodium story has become increasingly revealed,
until nothing less than a separate chapter will do justice to it even in a preliminary
statement, which is all that this can claim to be. In making it, however, I am on this
occasion drawing not merely on my own work but also on the collaborative effort of
my colleague Miss Davies, who has contributed very greatly to the elucidation of the
facts to be described below and without whom this chapter could not have been written.
The Common Polypody is a very familiar but at the same time a rather isolated fern.
It was well known to our ancestors, being mentioned in a number of medieval herbals
including the Ortus Sanitatis of 1491 (Fig. 126), in which the textual reference begins
127
POLYPODIUM VULGARE
with the words 'Polypodium latine, grece Dipteris . . .'. It is perhaps for this reason
that the popular name of 'Polypody' is still based on the Latin rather than on a folk
name in the vernacular as in other ferns, but it is probably only a coincidence that it is
still bracketed with Dipteris in the phyletic view of Bower (cf p. 89) as a somewhat
doubtful member of the 'Dipteroid' affinity, since Dipteris at present denotes a very
restricted genus confined to the Malay Peninsula and almost certainly unknown to the
herbalists.
The European Polypodium vulgare L., although the type species of the type genus of the
family Polypodiaceae to which all the ferns hitherto discussed except Osmunda have
until recently been held to belong, has no well-authenticated near relatives. The genus
is, in Bower's words, a comprehensive but phyletically confused one in which, owing to
the loss of the morphological characters of the indusium, derivatives of widely different
origin have been grouped together. Some of these have already been separated out and
discussed in the context of their nearest indusiate relatives {Phegopteris, Gymnocarpium,
Chapter 5; Athyrium alpestre and flexile, Chapter 6), but even without these we are left
with a large and almost exclusively tropical genus in which it is by no means clear where
exactly the one temperate species should be placed. Doubts about this have been
repeatedly expressed, and it is only by invoking a resemblance to the tropical Gonio-
phlebium that Bower has classified Polypodium vulgare at all. On the other hand, the idea
that there could be doubts about the specific integrity* of P. vulgare is one which only
a few specialists and no European writers of Floras appear ever to have entertained,
and therefore the surprise with which the unequivocal fact was revealed by the cytology
was very considerable.
'P. vulgare' is in fact a comprehensive name for a group of well-defined if closely
related species possessing an aggregate range which extends right round the northern
hemisphere together with South Africa, Kerguelen Island and perhaps Hawaii, to the
first two of which it could, however, have been introduced from Europe. Each species
within this range has a characteristic ecological or geographical area which only in
certain cases overlaps that of others. Moreover, the cytological differences are such as
to indicate quite clearly that populations of different ages are represented.
As was announced in a preliminary communication (Manton, 1947) there are three
distinct cytological types in Britain and the nearer parts of the continent of Europe.
The gametic chromosome numbers are 37, 74 and 1 1 1, which correspond therefore to
sporophytes of diploid, tetraploid and hexaploid constitution in a polyploid series on
37. The monoploid number itself has already been illustrated for a horticuhural variety
oi Polypodium known as P. vulgare var. semilacerum Hort. in Fig. 123a, p. 123, and it will
be seen again in Fig. 143 on p. 141. The gametic number of the tetraploid {n = 74)
may be seen in a specimen from Norway in Fig. 136, and that of the hexaploid (n= 1 1 1)
in Figs. 1 27-1 29. The first of the hexaploid figures is that reproduced in the prehminary
note in which an approximate count of c. 112 had been recorded earlier. That the
gametic number is 1 1 1 without any equivocation is, however, clear, among other thmgs,
from Fig. 128, in which the whole array of chromosomes is dispersed with such spec-
* Compare, for example, the views of Christensen (1928), which summarized excellently the prevailing
opinion with which this work began.
128
POLTPODIUM VULGARE
tacular clarity that the only difficulty in demonstrating the number with complete
finality is that the area occupied by the flattened nucleus is so large that an unusually
low magnification has had to be used in order to reduce the dimensions to that of the
printed page.
At the time of publication of the preliminary note it was not certain whether some of
these types might perhaps have been of horticultural origin. This matter is, however,
Fig. 127. Meiosis in hexaploid Po/v/'Ofl'n/OT from Windermere, fresh acetocarmine. x 1000. This was the
first specimen obtained and was illustrated in the preliminary note (Manton, 1947) at a lower
magnification. The chromosome number was at first thought to be 'c. 112', but is now known to
be III. Compare with Figs. 128-129.
no longer in doubt. All three are characteristic and well-estabhshed components of the
European flora over very large areas, and in the normal condition show none of the
peculiarities which constitute the horticultural monstrosities to which varietal names
have so often been given. Each has, however, a distinctive morphological character,
and their separation in the field is a matter presenting no difficulty except where genuine
admixture due to hybridization is occurring. There is, moreover, strong reason to
suppose that not only are they distinguishable by their chromosomes and morphology
but that they also have characteristically different ecological, or perhaps more correctly
climatic, requirements which are reflected in differences of geographical distribution.
Owing to obvious difficulties in sampling populations in central and eastern Europe
at the present time, it has not yet been possible to investigate distributions fully. The
compilation of a map will therefore be deferred, but such information as is available will
be included in the description of each species in turn.
MFC 129 9
«t ^ ^
./
»
0
«
4
► o
f
, ^
U -^
^
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4
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Fig. 128. Meiosis in hexa-
ploid Polypodium, perma-
nent acetocarmine. x 500.
A very clear preparation
showing n = 1 1 1 without
equivocation but needing
a low magnification for
reproduction. For ex-
planatory diagram see
Fig. 129.
* V
*■ * x <
^
4.
^^* ^ ^ ^xv*
Hexap/oid po/c^poc/ium ^ "
Fig. 129. Explanatory diagram to Fig. 128. x 750.
Fig. 130. Morphology of the three European species of Polypodium from Hving leaves grown under
comparable conditions, half natural size. a. The diploid (var. serratum), the left-hand leaf from
Cheddar (Somerset), the right-hand leaf from the south of France, b. A leaf from the same plant
as the right-hand specimen of a to show forwardly projecting lower pinnae, c. The normal form
of tetraploid Polypodium in Britain; note the narrow frond, d. A large and a small frond from the
hexaploid in Britain.
131 9-2
POLTPODIUM VULGARE
The commonest form of Polypodium in most parts of western Europe from the north of
Scandinavia to the Pyrenees is the tetraploid. This is shown photographically in
Fig. 130^ and in silhouette in Fig. 135 c. It possesses characteristically the narrow
outline for which the crude woodcut of the Ortus Sanitatis might be taken as a
rough diagram. Many details of the shape of the pinnae are variable, but two fairly
constant characters are the circular, as opposed to oval, sori and the number of indurated
cells in the annulus. Attention was first drawn to the usefulness of this last character by
Farquet in 1933 with special reference to P. vulgare var. serratum (Willd.) Milde to be
m
1^^ 'WX
' %
'1m
*«'
!S
^M%
5«.
[*'
^4w'
^^^
^N
Fig. 131. Polypodium pinnae enlarged to twice natural size to show the shapes of the sori. a. The French
diploid, sori oval. b. The tetraploid, sori round, c. The hexaploid, from a rather small frond,
sori oval.
described below, and our experience strongly confirms his. In tetraploid Polypodium
the range of numbers is from 11 to 13 cells with 1 2 as the commonest number.
A sporangium showing this is reproduced in Fig. 132.
In contrast to the tetraploid the diploid, which corresponds in our experience with
descriptions of P. vulgare var. serratum wherever we have found it, is of characteristically
southern affinities. It appears to be the only type present at low altitudes in the Medi-
terranean basin. Living material has been cytologically examined by us from two places
in the south of France (Pont-du-Gard and Perpignan) and from north Italy while
herbarium records extend the range at least as far as North Africa and the Atlantic
Islands (Madeira, Teneriflfe, etc.). In northern Europe it is found in discontinuous
patches, usually on limestone and often in districts containing other species of southern
origin. Examples of such localities from which we have already obtained it are the
132
POLTPODIUM VULGARE
Rhone Valley in Switzerland at the eastern end of the Lake of Geneva at an elevation
not exceeding 400 m., at Dartmouth in south Devon, abundantly in the Cheddar Gorge
and some other limestone habitats in Somerset, on Ingleborough in Yorkshire and in the
Burren in the west of Ireland. It may confidently be expected to occur in central
r-j-j
y
\
Fig. 132. Sporangium of tetraploid Po/;'/^o</2wnz from
Cumberland showing twelve indurated cells in the
annulus, from a glycerine jelly mount, x 100.
Fig. 133. Sporangium of diploid Polypodium
from Cheddar showing five indurated cells
in the annulus, from a glycerine jelly mount.
X 100.
Fig. 1 34. Sporangium of hexaploid Polypodium from Anglesey showing approximately nine indurated cells
in the annulus, from a glycerine jelly mount.
X 100.
Europe, and profitable places to look would be the Biscutella districts in the river valleys
of the Rhine, Elbe, Oder and Danube, but until these have been sampled the eastern
limits of this 'variety' cannot be determined.
The morphological characteristics of the diploid can best be appreciated by a glance
at Figs. I30fl, b, 131 a and 135 a. The most striking character is the oval frond which
even in small specimens widens out disproportionately much in the centre compared
with the tetraploid. Another detail which not all leaves display but which can generally
be found in some leaves on every plant is that the two lowest pinnae often project sharply
forward in the manner so characteristic of the Beech Fern {Phegopteris) . This character
133
POLTPODIUM VULGARE
is not always detectable when the leaf has been artificially flattened by herbarium
treatment, though it can sometimes be inferred by the crushed condition of the lowest
pinnae. Another detail which seems very constant is the shape of the sorus, which is
characteristically oval and not round. This character is more clearly visible in a young
leaf at or before the stage at which sporangia are ready for fixing than it is when the
spores are ripe. A very important character, since it directly affects the ecological
requirements, is the seasonal periodicity of the leaves. In the tetraploid, new fertile
leaves are put up in early summer (May or June in Britain, early July in Scandinavia)
and remain fresh unless battered to pieces at the end of the winter; the spores are ripe
in July or August. In 'var. serratum\ however, the principal dormant season is the
summer during which, in times of drought, the leaves may die away. New fertile fronds
only appear in autumn (August or September), and the spores are therefore ripe so
late that it may be suspected that in bad years in the more northerly habitats they may
perhaps fail to ripen at all. Lastly, the microscopic character of the number of cells in
the annulus is found to be extremely helpful; in all the localities listed above, the average
number of indurated cells is 5 with a total range from 4 to 6. A sporangium is
photographed in Fig. 133.
It may be suspected that the claims of var. serratum to be regarded as a separate
species would have been generally recognized long ago but for the existence of the
hexaploid which is almost certainly in origin a hybrid between diploid and tetraploid
and which therefore not unnaturally combines characters of both. Some leaves are
photographed in Fig. 130^?, and a silhouette appears in Fig. 135^. The hexaploid
is always a coarse plant and often very large. The leaves are thicker and fleshier
than either of the others, but their shape, size being discounted, is almost exactly
intermediate between diploid and tetraploid. In some details, however, the characters
of the diploid seem to show simple dominance. Thus the projecting lower pinnae
otherwise characteristic of the diploid are often also present in the hexaploid. The
sori likewise are oval, as in the diploid, but the number of indurated cells in the
annulus is exactly intermediate, being on an average 9 (with a range of 8-10), although
all the cells are distinctly larger than in either diploid or tetraploid. The shape of the
sori and the nature of the annulus are illustrated in Figs. 131c and 134. With regard to
seasonal periodicity the hexaploid differs from both the others in having an extended
season from summer to autumn, and it was therefore probably no coincidence that
when fixings were first attempted in September 1944 at Windermere in the Lake District,
a region in which tetraploids abound, it was only on a hexaploid that a young fertile
frond was found.
Ecologically and geographically the hexaploid seems to prefer a moister climate than
do either of the others. It is the commonest type in Ireland, Wales, south-west England
and the Channel Islands ; indeed in Jersey, where it is abundant all over the island, no
other form has so far been found. On the mainland of Europe its distribution is less
well known, though it is certainly present in coastal districts from Portugal to Holland
and inland it reaches the lower slopes of the Alps.
That the hexaploid has indeed originated from a hybrid between diploid and tetra-
ploid which has attained stability by doubling its chromosomes is suggested not only by
134
^**iii~^
o o
'o S
^ ?
13 ^
ffi s-
■^ -n "o
CO ;C >^
bb
POLTPODIUM VULGARE
the morphology but also by the breeding behaviour of all three types when their native
localities abut. Thus among the rather limited number of diploids collected at random
on journeys as occasion offered, no less than three cases of triploids have been included.
A leaf of one of these from the Rhone Valley in Switzerland is shown in Fig. 135^,
and the chromosomes of another, from the lower slopes of the Pyrenees near Perpignan
in Fig. 139. There is almost complete failure of pairing in the triploids, the shape of
the univalents making a very striking contrast with that of the normal appearance of
pairs in the putative parent species. These triploids, the third of which came from
north Italy, can hardly be other than hybrids between diploid and tetraploid, and they
could be the prototypes of the hexaploids before the chromosome number was doubled.
i*
i<
1
M
mjf
f
4
V
* ♦*.
%
h;
n
y
^
Fig. 136. Meiosis in tetraploid Polypodium from Norway to show the shapes
of the 74 bivalents for comparison with the triploid of Fig. 139. x 1000.
Where tetraploids and hexaploids grow together, pentaploid hybrids are to be found,
and some half-dozen of these have been included in our own collections made in different
parts of north and south England and as far afield as Holland. One leaf is shown in
Fig. 135^ and the chromosomes in Fig. 138. As may be seen in the diagram (Fig. 137)
there are exactly 74 pairs and 37 univalents.
This pairing is exactly what would be expected if the hexaploid is the allopolyploid
between diploid and tetraploid as postulated. The triploid indicates that the diploid
and tetraploid are sufficiently different from each other to have practically no chromo-
somes in common in spite of their readiness to breed together. On the other hand, the
perfect pairing of 74 of the chromosomes of the hexaploid when backcrossed to the
tetraploid indicates that the gametic complement of the tetraploid is present intact in
the hexaploid, and that the unpaired chromosomes in the pentaploids are therefore
almost certainly those of 'var. serratum\ which have already been seen to be non-
homologous with these by means of the triploid hybrid.
136
0 ,001 •l^
'iU'i^\
I
Fig. 137. Explanatory diagram to Fig. 138 with the 74 pairs in black
and the 37 univalents in outline, x 2000.
m
n
* -*. -.* ♦
Fig. 138. Meiosis in pentaploid Polypodium, from a wild hybrid from Holland, permanent acetocarmine.
X 1000. For explanatory diagram see Fig. 137.
POLTPODIUM VULGARE
The taxonomic conclusions for Europe would therefore seem to be that we are dealing
with three distinct though closely related species of which the third, the hexaploid, is
compounded of the other two and of very recent origin. The only point of uncertainty
concerns their names. There is apparently some doubt as to the nature of Linnaeus
type specimen since none is included in the Linnean herbarium, and it appears certain
that the name senatum could not be accepted as a specific epithet since it has already
been used for an entirely different species of Polypodium. This matter may therefore
perhaps be laid before professional systematists and decision deferred as to nomen-
clature, since to act otherwise incurs grave risk of encumbering the literature with
invahd epithets which might later have to be changed.
■■*■ •/ ■•• v.-'
Fig. 139. Meiosis in triploid Polypodium from a wild hybrid believed to be between tetraploid and
diploid. From the foot of the Pyrenees near Perpignan. Permanent acetocarmine. x 1000.
Almost all the chromosomes unpaired (3/2= 1 1 1).
There is, however, a little more to add to the cytological story. I have been fortunate
in receiving from friends and correspondents some living wild material from both sides
of North America. Fig. 141 shows a whole plant, natural size, of P. virginianum from
Nova Scotia. Its general resemblance to the tetraploid of Europe is striking, although
its identity as P. virginianum is attested by the possession of the very characteristic
paraphyses (Fig. 142) only encountered in eastern America. Cytologically this plant
is, however, diploid, and the same is true of specimens attributable to P. vulgare var.
occidentale Hook, received from Vancouver Island and the Rocky Mountains, one leaf of
which is visible in Fig. 140 and the chromosomes in Figs. 143 and 144. Since it is im-
possible to equate 'var. occidentale' with 'var. senatum' or P. virginianum with the
European tetraploid, it seems probable that we shall have to accept at least two
American species in addition to the three of Europe.
Lest, however, the reader should at this point lose patience thinking that only a
troublesome tangle of nomenclature is involved, it is perhaps worth pointing out the
extreme interest of the general situation which is beginning to become visible. In the
138
Fig. 140. ' Polypodium vulgare var. occidentale'
Hook. fromVancouver Island, from a living
frond grown in cultivation. Natural size.
Fig. 141. Polypodium virginianumh. fromNova
Scotia, from a dried frond of the plant
used, before cultivation. Natural size.
139
POLTPODIUM VULGARE
summer of 1948 I was able to discuss the world distribution of Polypodium with
Professor Hulten of Stockholm, who alone has adequate information regarding large
tracts of central and eastern Asia, and he has kindly given permission to publish the
gist of his verbal communication. This is that P. vulgare sens. lat. is to be found right
round the northern hemisphere in a total area which is not now continuous though it
may formerly have been so. The most conspicuous gaps are east of Lake Baikal in
central Russia, the whole of central China, central North America and the whole of
Fig. 142. Characteristic paraphyses diagnostic oi Polypodium virginianum, after Martens (1943)
Greenland. The primary cause of disruption of this kind is believed to be glaciation,
and the present distribution of diploid populations in Europe is in agreement with this.
If now the cytological facts so far obtained are superimposed on this general distribu-
tion we may interpret it as meaning that an ancient diploid stock must have spread
round the world, breaking up as it went into ecospecies. During periods of extermina-
tion some of these ecospecies survived in major refuge areas of which we may discern
at least four, namely, south Europe, eastern North America, western North America
(including perhaps ' Beringia ' as the unglaciated region of the Behring Straits has been
140
POLYPODIUM VULGARE
named by Hulten) and 'somewhere in Asia'. From these refuge areas under more equable
conditions our present taxonomic species have spread again, though in most cases the
areas occupied have not yet Hnked up with each other. An exception is, however, to be
found for the tetraploid of Europe which may perhaps be a fairly recent species and
which has certainly spread very vigorously over the heavily glaciated territories of
our continent. Where this new tetraploid has encountered the remnant of the older
population still persisting in the south it has hybridized with it, and the newest species,
the hexaploid, is the result.
What are the nearest ancestors of the tetraploid or where in Europe or Asia it can have
arisen can only be elucidated by further inquiry. We can likewise hope to investigate
i^ar. occ/c/en/-c7/e n ^ 37
Fig. 143. Gnxoxao^ovn&'i, oi Folypodium vulgare Fig. 144. Explanatory diagram to
var. occidentale from the Rocky Mountains. Fig. 143. x 1500.
X 1000.
the nature of the genetical differences which at present separate the various known
diploid species from each other. The present state of knowledge is thus clearly not the
end but the beginning of a problem which, if it can be unravelled, should illuminate the
migrations of floras, their origins and developments, not only in Europe but over the
whole of the northern hemisphere in the time lapse of a geological period. This hope is
also inherent in the problem of Cystopteris, but the additional complexities of that
genus may in the end impede progress irremediably. The relative simplicity of the
situation in Polypodium is likely to be its greatest asset, and it is no exaggeration to
say that even in the present state of knowledge Polypodium has turned out to be by far
the most interesting member of the British fern flora which we have so far encountered.
SUMMARY
Summarizing this chapter, it is perhaps sufficient to say that morphological and
cytological reasons have been given for recognizing three separate species of Polypodium
in Europe and at least two from the P. vulgare complex in America. A preliminary
discussion of geographical distribution is included. Some information regarding the
mutual relationships of the European species has been supplied from wild hybrids.
141
CHAPTER 9
THREE SPECIAL CASES OF FERN HYBRIDS:
SCOLOPENDRIUM HTBRIDUM, WOODSIA
AND POLTSriCHUM ILL TRIG UM
As a supplement to the foregoing account of the British fern flora attention may profit-
ably be given to a few special cases of species hybrids of non-British origin which,
nevertheless, add appreciably to our knowledge of species or genera already considered.
Scolopendrium hybridum Milde [ = Phyllitis hybrida (Milde) Christensen) (Fig. 145) is
Fig. 145. Scolopendrium hybridum Milde. Natural size. a. From a dried leaf. b. From a living leaf
of the next generation. Both grown in cultivation.
Fig. 146. Scolopendrium hybridum WMc. Half natural size. From Milde's original
drawing after Luerssen in Rabenhorst's Kryptogamenjlora (1889).
a somewhat problematical plant with a very restricted range. It is endemic to five small
islands in the Adriatic Sea near the Dalmatian coast known as the Quarnero Group,
and consisting of Lussino, Osiri, Arbe, Dohn and S. Gregorio. The first specimen known
to science (cf Fig. 146) was discovered on Lussino by Reichardt in 1862, who noticed
it growing in a wall among a dense population oi Ceterach qfficinarum. Reichardt's speci-
142
SCOLOPENDRIUM HTBRIDUM, WOODSIA AND POLYSTICHUM ILLYRICUM
men was described in 1 864 and named by Milde, who regarded it as a hybrid between
Ceterach (Fig. 1470) and Scolopendrium vulgare, and since the latter species was not at that
time recorded from the island, Milde predicted that it would subsequently be found.
Another suggestion made later by Luerssen (1889) was that a more probable affinity
might be with the south European S. hemionitis Lag., Garcia & Clem. (Fig. 147^).
This species has a much more restricted range than S. vulgare and is the only other
member of the genus Scolopendrium present in Europe; it occurs on many of the Mediter-
Fig. 147. Two of the imagined parents of Sco/o/^m^nwrn/i^iri^Mm. Natural size. a. Ceterach officinarum DC.
from the south of France with 71=72. From a Hving leaf grown in cultivation, b. Scolopendrium
hemionitis Lag., Garcia & Clem, from the south of France, from a pressed wild leaf of a small plant.
ranean islands and south European coasts. An affinity with S. hemionitis has been
accepted as probable by most subsequent writers, and S. hybridum was indeed regarded
as a subspecies of 5". hemionitis by Ascherson and Graebner in 1896. The claim that it
should be regarded as a totally distinct species had, however, already been made*
by Heinz in 1892, and this may also have been suspected by Reichardt himself who
comments (1863) on the fertility of the spores. Most subsequent discussion in the litera-
ture has accepted this view, while reiterating the probability that the species may have
originated as a hybrid which had become fertile. Many lines of evidence have been
quoted. In addition to the morphological comparisons introduced by Milde anatomical
comparisons between Scolopendrium and Ceterach carried out by Hoffman in 1899 showed
Scolopendrium hybridum to be intermediate. A similar result was obtained by comparison
of the prothalli in 1922 by Howat, while at least two investigators have studied
* A clear summary of the history of this controversy will be found in Morton (1914a, 1925)-
THREE SPECIAL CASES OF FERN HYBRIDS:
the range of form to be found among natural populations of sporophytes. Thus Haracic
in 1893, who appears to have been the first person to study natural populations of the
plant, described three different morphological types and figured two of them under the
names of forma typica, var. Reichardtii (Milde's original specimen) and var. lobata.
Some additional evidence was added by Ivancich in 1923, who reinvestigated the ranges
of form and spore fertility among natural populations. Haracic's var. lobata was not
examined again, since it had been found only on the island of Osiri which Ivancich
did not visit. Haracic's forma typica was, however, described as the most stable, and
his var. Reichardtii as the most unstable. In addition, three other varieties were described,
namely, var. hemionitifolia, which closely resembled Scolopendrium hemionitis, var. erosa
which differed from forma typica in having large basal lobes and more irregular outline
to the rest of the lamina, and lastly var. ceterachifolia which very closely resembled
Ceterach. This last 'variety' was in effect only one individual plant, since it was sterile
from spores and could only be propagated from the rhizome. By means of it, however, a
series of fronds could be shown which almost entirely bridged the gap between one genus
and the other.
This history has been given in some detail partly because Scolopendrium hybridum is
somewhat unfamiliar to most botanists, but also because it differs from all the other
suspected fern hybrids in combining characters not merely of two different species
but of two different genera whose close relationship might not otherwise be suspected.
It is thus clearly a plant of quite unusual interest, and I am extremely fortunate in
having had access to living material of it. This has been of several kinds, all, however,
ultimately referring to the one island of Lussino. The first material to reach me con-
sisted of a pot of tiny sporeling plants raised by the late Dr F. W. Stansfield and presented
to me after his death by his son. The origin of the pot was described by Stansfield in the
British Fern Gazette (vol. vii, p. 91, 1936) as follows: 'A spore-bearing frond was sent to
M. Kestner [of Lausanne] from the island of Lunin [accidental misreading of Lussin].
M. Kestner brushed off some spores and raised plants from them and, mainly from that
fact, he formed the opinion that it was not a hybrid but a species per se. He afterwards
sent on the frond to me and I was able to collect from it a few more spores which
I sowed and have now about a dozen tiny plants as the result.' These plants grew to
maturity and were the main source of cytological material. They were, however,
supplemented, shortly before the war, by two consignments of adult plants very kindly
suppHed by Professor Lona of Trieste. The first consignment consisted of one plant of
forma typica which had been in cultivation in Trieste Botanic Garden, and later, in
1938, some additional plants of various morphological kinds, though not including the
extremes (var. hemionitifolia and var. ceterachifolia), were sent direct from the island of
Lussino. Owing to the outbreak of hostilities the cytological study of this material was
less exhaustive than it would otherwise have been ; the fresh collections were, however,
invaluable in confirming the vahdity of the results obtained with the Stansfield material.
During the war itself almost the entire collection was lost, but spores saved from the
last surviving individual have re-estabHshed the culture. A silhouette of the dry spore-
bearing frond, together with that of a living leaf of the next generation obtained from it,
have already been shown in Fig, 145 a and b. They resemble somewhat the description
144
SCOLOPENDRIUM HTBRIDUM, WOODSIA AND POLYSTICHUM ILLTRICUM
of Ivancich's var. erosa, though the general resemblance to Milde's type specimen
(Fig. 146) is equally obvious. Comparable fronds of the putative parent species,
Ceterach and Scolopendrium hemionitis, both from the neighbourhood of Marseilles, are
illustrated in Fig. 147.
The cytological result which has been obtained from all this material is that S. hybri-
dum on the island of Lussino is a tetraploid in comparison with the normal S. vulgare.
Owing to the vicissitudes of war and the delicacy of the plants, the demonstration of
chromosome number lacks the elegance which is attainable with the squash techniques,
but the difference in size of the metaphase plates at meiosis can at once be seen by
comparing Fig. 148^ and c. A similar comparison of mitotic cells can be made from
Fig. 148. Chromosomes oi Scolopendrium hybridum Milde with S. vulgare Sm. for comparison, from sections.
X 1000. a. Root of 5. A^ji^nWum forma (y/^zVa from Trieste Botanic Garden. 2n=i^4.. b. Meiosis
in the plant of Fig. 145. n = c. 70 (probably 72). c. Meiosis in S. vulgare for comparison. ^ = 36
(cf. Chapter 7).
Fig. 148 fl. Final accuracy is unfortunately not readily obtainable from sections, but the
approximate results for S. hybridum are all of the order of 140 chromosomes for roots and
c. 70 at meiosis. Since the exact chromosome number of S. vulgare is known to be
n = 36 (Chapter 7), the exact gametic number for S. hybridum is almost certainly n = 72.
The detection of tetraploidy makes a hybrid origin for S. hybridum more rather than
less probable, though the parentage is by no means self-evident. Ceterach officinarum,
the only European species of that genus, is also a tetraploid (cf. Chapter 6) with n = 72,
both in Britain and in France, and any direct hybrid with Scolopendrium vulgare would
be triploid at first and hexaploid after chromosome doubling. With regard to
S. hemionitis, however, there has hitherto been no cytological information.
After many years of fruitless efforts to obtain living material of S. hemionitis I was
fortunate in 1946 to be able to visit the Mediterranean coast near Marseilles and to
collect some spores and adult plants of this species. A leaf of a small plant at the time of
collection is reproduced in Fig. 147^, with a larger leaf produced from the same plant
two years later in a warm house at Kew in Fig. 149. The apparent fimbriations of the
margin in the latter specimen are due to undulations of the leaf lamina produced, no
doubt, by the somewhat abnormal growing conditions. The very characteristic shape
of the auricles is, however, fully displayed in both specimens, and both have also the
fleshy texture characteristic of this rather uncommon species.
The chromosomes of ^. hemionitis are illustrated in Figs. 150 and 151. Fig. 150^
shows mitosis in a root with c. 70 (actually no doubt 72) chromosomes. Figs. 150a and
MFC i^^ 10
THREE SPECIAL CASES OF FERN HYBRIDS:
151 represent meiosis in which the number is quite certainly n = 36. S. hemionitis, Hke
S. vulgare, is therefore a diploid species, and both contrast equally strongly with S. hybridum.
At this point it is greatly to be regretted
that the programme of experimental work
planned for this species but interrupted by
the war has not yet been resumed. The
L«l^
a b
Fig- 1 50- Chromosomes of Scolopendrium hemionitis Lag. ,
Garcia & Clem., from sections, x 1000. a. Meiosis.
n = 36. For explanatory diagram see Fig. 151.
b. Mitosis in a root. 2n='j2. For comparison with
S. hybridum see Fig. 148 a.
S hemlonlNs n^36
Fig. 151. Explanatory diagram to
Fig. 150 a. X 1500.
origin of S. hybridum is therefore still un-
certain, and, with the experience of Dryopteris
remota and Asplenium germanicum in our minds
it would be unwise to predict a conclusion
before the evidence is fully assembled. We
have three suggested parents to consider.
The two species of Scolopendrium are both suit-
able in chromosome number, but whether
either is actually related can only be deter-
mined by crossing S. hybridum with each and
examining chromosome pairing in the tri-
ploids so obtained. There remains, however,
Fig. 149. Scolopendrium hemionitis Lag., Garcia & Clem.
Living leaf of the same plant as Fig. 147^, grown in
cultivation showing characteristic auricles. Natural
size. For description see text.
146
SCOLOPENDRIUM HYBRIDUM, WOODSIA AND POLYSTICHUM ILLYRICUM
Ceterach. This cannot lightly be dismissed, even though an intergeneric hybrid seems at
first sight improbable and the chromosome number hitherto found {n = 72) is too high.
We cannot, however, assume that a form of Ceterach with a lower chromosome number
may not exist, and the fact that if the known number were halved it would be identical
with that of diploid Scolopendrium may indicate a closer relationship than one might
otherwise have expected to find. Further investigation of this problem is therefore very
much to be desired.
The second case to be discussed, that of Woodsia, came to my notice almost accident-
ally during the summer of 1948, and a full investigation has been impossible to carry
Fig. 152. Hybrid Woodsia. Series of leaves from the island of Runmaro near Stockholm, each from
a different plant, grown in cultivation. Natural size. For description see text.
out before going to press. Even in a preliminary form, however, the results are so
surprising to a British botanist that reference to them seems profitable.
The extreme rarity and alpine associations of W. ilvensis and JV. alpina in Britain have
already been commented upon in Chapter 7, and it is therefore at first an unfamiliar
experience to find both species growing, often in great profusion, at sea-level in Scan-
dinavia. Once this experience has been gained it is perhaps not so surprising to learn
that both in Sweden, Norway and also in Alaska (Hulten, 1941) both species can some-
times be found together and that in some of these localities hybrids are formed. One of
the best known of such places is the island of Runmaro in the archipelago near Stockholm
which was described in detail by Qyarfort in the Svensk botanisk Tidskrift for 193 1 (vol. xxv,
p. 36). I was not able myself to visit the island, since in 1948 it was still included in
147
10-2
THREE SPECIAL CASES OF FERN HYBRIDS:
a military defence area forbidden to foreigners. At my request, however, Professor Halle
of Stockholms Riksmuseet very kindly undertook to send out two members of the museum
staff (G. Haglund and R. Rydberg), who visited the island on my behalf and brought back
a very large and varied collection of living plants. A selection of these was dispatched by
f
»
Fig. 153. Meiosis in hybrid Woodsia, permanent acetocarmine preparations, a. Anaphase of first
meiotic division from the leaf of Fig. 1 52 Z>, showing lagging univalents, x 1000. b. Anaphase of
second meiotic division showing lagging halves of univalents, from the leaf of Fig. 152 c. x 1000.
c. First meiotic metaphase from the leaf of Fig. I52</. x 1500. For explanatory diagram see Fig. 154.
air to Kew where they throve amazingly, and one month after arrival there, namely, at
the end of August 1948, a new crop of fronds had been put up on every one, from which
fixations and the following information were obtained.
Fig. 152 shows the range of leaves represented in this collection. If comparison is
made with Fig, 107, p. no, it will be clear that the leaf of Fig. 152 a is most hke that of
148
SCOLOPENDRIUM HTBRIDUM, WOODSIA AND POLYSTICHUM ILLYRICUM
W. ilvensis though somewhat coarse and flaccid, as greenhouse fronds are apt to be.
Fig. 152^ is the most alpina-like leaf, and Figs. 152 b-d are of the intermediate type which
suggests hybridity.
Fixations from all these leaves and from some others were made, and though none
was sufficiently perfect to establish the chromosome numbers in full detail had the
genus been unknown, the information presented in Chapter 7 is sufficient to make them
interpretable. The plant of Fig. 1520 had a reduced chromosome number of approxi-
mately 40 and few if any unpaired chromosomes ; it therefore resembles W. ilvensis in
8 -
o
cb
0
o
0° 0.
o'
0
u
a
iO o
Hijbricl UJoodsia 3n -- c. 123
Fig. 154. Explanatory diagram to Fig. 153 c, probable univalents in outline,
probable pairs in black, x 2000.
Britain. The plant of Fig. 152^ had no unpaired chromosomes and approximately
80 bivalents at diakinesis ; it therefore approximates to and is probably identical \vith
the normal state of W. alpina in Britain for which n = <;. 82. In each of the plants of
Figs. 152 b-d on the other hand numerous unpaired chromosomes were present. Some
characteristic anaphase figures showing lagging univalents at both meiotic divisions are
reproduced in Fig. 153 a and b, while Fig. 153^ gives a polar view of a spread first
meiotic metaphase, in which an approximate though not an exact count can be
made. An explanatory diagram is given in Fig. 154, which may perhaps help the
reader to distinguish between pairs and univalents. In some parts of the figure the
groups are too closely crowded to be fully analysed, but there is no doubt that pairs and
univalents are present in almost equal numbers and that the approximate number for
each is of the order of 40. Since we know from Chapter 7 that, in Woodsia, n=c. 41 this
149
Fig. 155. Polystichum illyricum Hahne, living fertile
leaf from Switzerland grown in cultivation,
somewhat depauperate. Natural size.
150
SCOLOPENDRIUM HTBRIDUM, WOODSIA AND POLTSTICHUM ILLTRICUM
is sufficient demonstration that some at least of the putative hybrids are triploids, as
they should be if the interpretation of their nature has been correct.
The relevance of this to our general field of inquiry is close. In spite of the imper-
fection of the cytology at this preliminary stage we are undoubtedly seeing a type of
behaviour which has much in common with the Male Fern story of Chapter 4. As in the
case of the artificial hybrid between Dryopteris abbreviata and D. Filix-mas, we have a
triploid in which two gametic sets of chromosomes have paired with each other and one
has remained unpaired. In the case of Woodsia the gametic sets which have paired
seem necessarily to be those of W. ilvensis, and we therefore reach the somewhat
surprising conclusion that the diploid species IV. ilvensis must be part-parental to the
tetraploid species W. alpina in the same sense that Dryopteris abbreviata has shown itself
to be part-parental to the Male Fern.
Woodsia alpina seems therefore to be another member of the British flora for which a
hybrid origin must be assumed, though, as in the case of the Male Fern, we know one
parent but not the other. That W. glabella, the third European species, may perhaps be
the other parent of W. alpina is quite possible though the fact can only be determined
by experiment. This problem also must therefore be left undecided until some future
occasion.
The last case to be discussed in this chapter is that oi Polystichum illyricum Hahne and
related forms. This is the name given to the putative hybrid between P. aculeatum and
P. Lonchitis. As already pointed out in Chapter 6 these two species in Britain occupy
rather different habitats, the Holly Fern, P. Lonchitis, being almost always an alpine
while P. aculeatum is a lowland or at most a montane rock plant. Under most normal
conditions they do not therefore encounter one another; occasionally, however, this
occurs. In Britain the only locality known to me where this happens is in the limestone
pavement of the Craven area in the northern Pennines, and comparable admixture
has been described from time to time from various parts of the Continent. Only
in such places is P. illyricum to be found. It is represented by single individuals
(as opposed to homogeneous populations), which betray their hybrid nature both by
the possession of morphological characters (cf. Fig. 155) intermediate between those of
the putative parents and also by marked spore sterility.
The material of P. illyricum available to me was obtained on each of two visits
made in 1937 and 1947 respectively to one of the classic localities for this particular
mixture of species, namely, that adjacent to the alpine garden at Pont-de-Nant above
Bex in the Rhone Valley in Switzerland. I am greatly indebted to the authorities of the
University of Lausanne for facilitating both visits.
The locality in question is a Picea wood on the south-facing slopes of a tributary valley
at an altitude of 4100 ft. The soil is calcareous and the floor of the wood is composed
of gigantic boulders, partly moss covered, in the cracks under which Polystichum
Lonchitis, P. aculeatum {P. lobatum), together with putative hybrids, grow in great pro-
fusion and in very close proximity to each other. Some characteristic old plants of all
three types had been transferred to the alpine garden some years before my visit, and
fixings of sporangia were made on all of these. Other plants of all three types were
collected on both occasions and posted alive to England for further study.
151
THREE SPECIAL CASES OF FERN HYBRIDS:
Investigation of the purest forms of the parent species in the alpine garden showed
that the cytological nature of these was exactly as in England (cf. p. 92), P. Lonchitis
being a diploid species and P. aculeatum tetraploid. This may perhaps be sufficiently
demonstrated by Fig. 157a and b, which are from the original fixings made on the
journey in 1937. These fixings are very inferior in quality to those otherwise obtain-
able, but as an addition to what has previously been given in Chapter 6, they will
perhaps serve as a rough demonstration that the two species on the Continent, as in
Britain, dififer in chromosome number. This circumstance is a very fortunate one,
since a simple root-tip count should confirm or refute the correctness of the diagnosis
Fig. 156. Series of pinnae from dried wild fronds of different adult plants from one locality (Les
Plans sur Bex, Switzerland) to show range of form of hybrid Polystichum. a. P. Lonchitis (L.) Roth.
b, c. P. illyricum Hahne. d. P. aculeatum (L.) Roth, ( = P. lobatum (Huds.) Woynar).
of the putative hybrid, for P. illyricum, if it really is the interspecific cross between
P. Lonchitis and P. aculeatum, ought to be a triploid with 123 chromosomes in its roots.
Among the plants suspected to be P. illyricum on morphological grounds three were
shown to be triploid by root-tip counts. It had, however, been at once apparent from
scrutiny of the living populations before collection that a considerable range of morpho-
logical types grading between the pure forms of the putative parents could be found,
and since a number of these had also been sent to England it was not surprising to find
that not all were actually triploid. In the 1937 visit one suspected hybrid, rather closer
than the average to P. aculeatum, turned out to be a tetraploid, while on the 1947 visit
two plants rather closer to P. Lonchitis than usual, though with more deeply cut-up
pinnae, proved on examination to be diploid or nearly so. Some examples of the range
of pinnae found on the 1947 visit are shown in silhouette in Fig. 156, in each case the
pinnae chosen being from large fertile fronds. The parental types, together with diploid
and triploid putative hybrids, are represented, and they obviously make an almost
continuous morphological series.
152
SCOLOPENDRIUM HTBRIDUM, WOODSIA AND POLTSTICHUM ILLTRICUM
The interpretation, of this series is fairly straightforward. The presence of the triploids
is in itself sufficient evidence for the correctness of the original diagnosis of the cross.
The presence of the others seems therefore necessarily to mean that this hybrid when first
formed is not completely sterile. There is no direct evidence of the nature of the
descendants which would be produced, but the behaviour of autotriploid Osmunda is a
helpful indication. If the expected proportion of balanced spores with the even poly-
ploid numbers can be formed, they would certainly be viable and the reversion to the
parental chromosome numbers would therefore not be surprising. That some signs of
hybridity still persist in such progeny would merely seem to imply that some extra
chromosomes belonging to one or other species are still present or that a measure of
gene exchange can occur between the chromosomes of the two species.
#
1^
4 *
--■■cr
m
■^
1
t
^ ' ^
i
V.
^
.-*■
■•#
**«^
J
b c
Fig. 157. Meiosis in Polystichum illyricum Hahne and its parents, from sections, a. P. aculeatum (L.) Roth,
wild fixation, x 750. b. P. Lonchitis (L.) Roth, wild fixation, x 750. c. P. illyricum Hahne,
grown in cultivation, good fixation, x 1500.
Be that as it may, an important taxonomic deduction follows from the study of
meiosis in the triploid plants. The evidence on this is presented in Figs. 157^, 158 and
159. Fig. 157^ shows meiosis in a triploid plant from the 1937 collection which had been
sent to England and grown on; exactly comparable results were also obtained in that
year from a hybrid plant fixed in the garden at Pont-de-Nant. Paired and unpaired
chromosomes are present in both in almost equal abundance, and the formation of
trivalents is so inconspicuous as to be negligible. More completely analysable evidence
is presented in Figs. 158 and 159 from acetocarmine preparations from one of the 1947
triploids which had been sent to England and fixed the following year. This does not
resemble autotriploid Osmunda, but again agrees very closely with the triploid hybrid
between Dryopteris Filix-mas and D. abbreviata. The conclusion which appears necessarily
to follow from this is that Polystichum aculeatum and P. Lonchitis are related in a way
comparable to Dryopteris Filix-mas and D. abbreviata, namely, that Polystichum aculeatum
is an allotetraploid and P. Lonchitis is one of its parents.
153
THREE SPECIAL CASES OF FERN HYBRIDS
This was somewhat unexpected, and the problem of the other parent is immediately
raised. As far as the British flora is concerned there is here little choice, since the only
other species is P. angulare (Kitaib.) Presl {P. setiferum (Forsk.) Woynar*), which, as
already explained (cf. p. go), was for many years regarded as co-specific with P. aculeatum
and which certainly resembles it more closely in the adult state than does P. Lonchitis.
Fig. 158. Meiosis In Po/yj^zVAwm tV/yncwm Hahne, permanent acetocarmine. x 1000. a. Diakinesis with
approximately 41 pairs and 41 univalents. For explanatory diagram see Fig. 159. b. First meiotic
metaphase showing total number of approximately 82 bodies.
5^a ^.
0^ oo
P illi/r/cum n = 4/
Fig. 159. Explanatory diagram to Fig. 158 a. x 1500. Pairs in black, univalents in outline.
To test this idea the attempt was made to hybridize P. aculeatum with P. angulare.
hs, parental stocks P. aculeatum from north Italy was used as a source of archegonia and
P. angulare from Dartmouth as a source of sperm. The hybrid (Fig. 160) proved easy to
* According to the International Rules the valid name for this species is P. setiferum. The retention
of the older name of P. angulare for the purposes of this chapter is merely a temporary expedient to main-
tain consistency with the scheme of nomenclature indicated in the Preface.
Fig. 1 60. Leaf of hybrid between Polystichum
aculeatum (L.) Roth and P. angulare (Kitaib
Presl for comparison with P. illyricum Hahne
(Fig. 155) and the parent species (Figs. 76
and 77, Chapter 6).
155
THREE SPECIAL CASES OF FERN HYBRIDS:
make and a number of examples were attested as such by a triploid chromosome count.
In appearance they resemble pure P. aculeatum so closely that their hybrid nature might
escape detection in a herbarium specimen unless the spores were examined and seen
to be abortive. Meiosis is, however, characteristic. The first sporangia were produced
in 1947, and the plants were fully fertile in 1948. Figs. 161 and 162 show a sample cell.
As expected, pairing closely resembles that of the other triploid. In the figured cell
there appear to be 41 pairs and 40 identifiable univalents. Since n — /\.i this is sufficiently
close to expectation to confirm the suggested close relationship of P. angulare with
P. aculeatum.
If this conclusion is correct it ought therefore to be possible to resynthesize P. aculeatum
by crossing P. angulare with P. Lonchitis followed by colchicine treatment. It is hoped
%
%.
.;%
•tfc
# r^"
%
§h ^
mm
w^
I Hr
. ^.
>'• *
J
• -^
^
m
0 •*•
>
m
it%^ -'
t
•
1 '^
«Mlk
•1
•
§
mt'
//""
Fig. 161. Meiosis in the triploid hybrid between Polystichum aculeatum (L.) Roth and P. angulare (Kitaib.)
Presl (Fig. 160), permanent acetocarmine. x 1500. For explanatory diagram see Fig. 162.
that this will indeed be done, and that the resulting plant will in fact resemble
P. aculeatum. If it does not it would be necessary to look at other diploid species of the genus
sufficiently close to the two in question to be likely to have chromosomes homologous
with these. To pursue this matter further at the moment, however, is impossible, since
even the synthesis from known parents would take at least 5 years to carry out and test.
The importance of P. illyricum in a general inquiry such as this is, however, obvious.
It has provided a clue to the genesis of yet one more polyploid hybrid species, and by
doing so has completed the analysis of a common British plant in terms not of suspected
relationship or certain affinity on one side only, as in the case of the Male Fern and
Woodsia, but with very strong probability indeed with respect to both the parents of the
hybrid species. This degree of completeness is a very welcome addition to the analyses
already discussed in this and previous chapters.
156
SCOLOPENDRIUM HTBRIDUM, WOODSIA AND POLYSTICHUM ILLTRICUM
^
^^
<^
/7=^/
i»
>0
0
' 1/
Triploid h^bric/
Fig. 162. Explanatory diagram to Fig. 161. x 2000. Pairs in black,
univalents in outline. One univalent has not been identified.
SUMMARY
Three non-British wild hybrid ferns have been described.
(i) Scolopendrium hybridum Milde, endemic to some Adriatic islands, is shown to be a
tetraploid. One possible parent, the south European S. hemionitis Lag., Garcia & Clem.,
is shown to be diploid as expected, but nothing is yet directly known about the relation-
ship.
(2) Woodsia. Some Swedish hybrids between W. ilvensis (L.) R.Br, and W. alpina
(Bolton) S. F. Gray have been shown to be triploid and to have chromosome pairing of a
type which suggests that the diploid species, W. ilvensis, is part-parental to the tetraploid
W. alpina.
(3) Two collections of Polystichum illyricum Hahne from Switzerland have been shown
to contain triploids which indicate by their chromosome pairing that P. Lonchitis (L.)
Roth is part-parental to P. aculeatum (L.) Roth. The other possible parent of P. aculeatum
is thought to be P. angulare (Kitaib.) Presl (P. setiferum (Forsk.) Woynar), and this is
confirmed by chromosome behaviour in triploid hybrids between these two species. The
synthesis of the tetraploid P. aculeatum from a hybrid between P. Lonchitis and P. angulare
is therefore awaited.
157
CHAPTER 10
APOGAMOUS FERNS. THE GENERAL
PHENOMENON
It may now perhaps be appropriate to consider a little more closely some of the remark-
able cytological peculiarities accompanying obligate apogamy in the Polypodiaceous
ferns. Some examples of this type of life history have already been introduced incident-
ally when describing Dryopteris Borreri, Phegopteris and others, but the peculiarity is not
confined to members of the Dryopteroid affinity. It has been met with from time to
time in widely scattered genera, in each of which it must have arisen de novo, yet the
main characteristics wherever they occur are so similar that all the known cases may
profitably be dealt with together.
The characteristics of this particular type of life history are that in spite of the
existence of normal-looking and fully functional spores, the prothalh which develop
from them are devoid of archegonia though antheridia may be, and usually are,
present in abundance. A sexual fusion takes no part in the initiation of the new sporo-
phyte which is formed directly from the central tissue of the prothallus at a stage of its
development which, in a sexual gametophyte, would just precede the thickening of the
central cushion. The first leaf of the new sporophyte is in most cases of a more adult
type than is the first leaf of a sexually produced young plant, but otherwise the only
additional external difference that can easily be detected is that the sporangia from
which the functional spores are derived contain 32 spores instead of the customary 64.
A list of species in which this type of life history is known to me is as follows :
Pteris cretica L.
Cyrtomium falcatum Presl
C. Fortunei ] .Sm. = ' Aspidium falcatum'
C. caryotideum (Wall.) Presl
Dryopteris Borreri Newm. = ' Nephr odium pseudomas' and also many varieties referred
to 'Z). Filix-mas'
D. remota (A.Br.) Hayek
D. atrata (Wall.) Ching = ' Nephrodium hirtipes'
Phegopteris polypodioides Fee
Pellaea atropurpurea (L.) Link
Asplenium monanthes L.
All of these species have been available to me for study, and though the hst will
certainly be found to be incomplete with regard to ferns as a whole, most of which have
never been examined from this point of view, the series is sufficiently wide to be a fair
sample of the general phenomenon. That many species should be reviewed by one
person is perhaps of importance in that a number of incomplete accounts of single
species have been produced at various times since the discovery of apogamy by Farlow
158
APOGAMOUS FERNS. THE GENERAL PHENOMENON
and de Bary at the end of the last century, and the available literature on the subject
is in a rather unsatisfactory state. From the cytological point of view there is, in my
experience, only one existing account which can be accepted as adequate, namely, that
by Dopp (1932) on Dryopteris remota. Other well-known and much-quoted papers, e.g,
Farmer and Digby (1907) on ' Nephrodium pseudomas wslts. polydactyla'' Wills and Dadds,
Allen (19 1 4) on ' Aspidium falcatum\ Steil (19 19) on ' Nephrodium hirtipes\ while con-
taining some correct observations are so seriously incomplete as to be actively mis-
leading.
The central cytological problem posed by all these plants is that of reconcihng the
absence of a sexual nuclear fusion with the presence of an apparently normal meiotic
process in the development of the spores. It is obvious that some compensating process
must exist at some point in the life cycle to stabilize chromosome numbers and to
prevent the progressive diminution which a repeated succession of meioses would other-
wise very rapidly bring about.
Some authors have looked for this compensating process in the gametophyte, and
Farmer and Digby (1907) in particular believed that they had found it when they
detected nuclear migrations in the vegetative cells of some of the prothalli o{ ' Mphrodium
pseudomas ( = Dryopteris Borreri) var. polydactyla', and interpreted this as 'pseudo-fertiliza-
tion'. The observational evidence for the existence, under certain circumstances, of
nuclear migrations may be accepted as valid, but no demonstration was offered of
either nuclear fusion or the origin of sporophytic tissue from cells with a higher chromo-
sorfle number than the rest of the prothallus. At that date, indeed, it is doubtful
whether cytological technique was sufficiently advanced to permit of a numerical
demonstration of this kind, and certainly these authors, having examined meiosis,
were unable to detect any numerical or other peculiarity about the process. Since in
fact the vars. 'polydactyla'' differ in no way from the other cases, in all of which, as will
shortly be seen, the compensating process looked for is in the sporangium, it is to be
hoped that 'pseudo-fertilization' after passing as a fact for a quarter of a century will
shortly cease to be quoted.
All subsequent authors have correctly concluded that the process which compensates
for the apparently normal meiosis is to be looked for in the sporangium. That the
process itself was not at once discovered may perhaps be explained in part by the un-
expected complexity of the sporangial development, and also probably in part by the
fact that all the earlier workers appear to have been consciously looking for a pseudo-
sexual process and were therefore predisposed to find it, in spite of incompleteness,
now obvious, in the evidence before them.
The first serious investigation of sporangial development in an apogamous fern was
by Allen in 19 14 on ' Aspidium falcatum\ now more correctly known as Cyrtomium
falcatum. She observed, correctly, that sporangia could be found with eight spore mother
cells and with sixteen spore mother cells, and from this she concluded prematurely that
one type turned into the other by fusion of the mother cells in pairs, a view which was
apparently supported by the detection of occasional intermediate stages. Allen's
evidence on the chromosome numbers was very seriously defective, as will be shown
below, but she deduced, correctly, that the meiotic process, though normal in
159
APOGAMOUS FERNS. THE GENERAL PHENOMENON
appearance, nevertheless produced spores with the same chromosome number as the
parent plant and that sporophyte and gametophyte were identical in nuclear content.
The correct interpretation of Allen's intermediate fusion stages was first given by
Steil in 1919 working on ' Nephr odium hirtipes' now known as Dryopteris atrata. Steil's
work was favoured by the fact that in Dryopteris atrata these ' intermediates ' are unusually
abundant, being, in my experience, more numerous in this species than any other type
of sporangium. At first, in a preliminary note, Steil accepted Miss Allen's view that
these were signs of nuclear fusion, but in his fuller account (1919) he realized that they
were better interpreted as signs of incomplete division, and that the true nature of the
compensation process was an incomplete nuclear division immediately preceding
meiosis, by means of which the nuclear content of the spore mother cells is momentarily
doubled.
Steil's account is incomplete in that he did not fully reahze the extent of the variation
in possible sporangial developments within one sorus, which is indeed less obvious in
D. atrata than in other species, and this is where Dopp's account of Z). remota (1932) is
greatly to be preferred. While authenticating the reality of an incomplete nuclear
division immediately preceding meiosis as the basic abnormality without which the
continued reproduction of the species would be impossible, Dopp showed clearly that
the abnormality is only effectively accomphshed in a proportion of sporangia, the
remainder being affected by differences of detail, most of which result in abortive or non-
viable spores. In Dopp's account three distinct types of development were recognized,
and though in most species the number can be extended to four, for most purposes his
account of Z). remota is adequate. In his last paper to appear before the war Dopp (1939)
extended his observations to 'D. Filix-mas var. cristata Hort.' and to the two poly dacty las,
and showed that they agreed exactly with D. remota in essentials.
The account which will be given here is in no sense based upon Dopp's work, but is
the resuh of an independent investigation begun (cf Manton, 1932) in the early 1930's
and continued in the first instance in ignorance of the parallel observations being
carried out in Germany. This should mean that something like finality may now be
claimed for the straightforward descriptive facts, as far as these go, for Dopp's observa-
tions and those to be described below confirm and supplement each other where they
relate to similar material, while the present work also extends the description to a
number of additional species. Since the actual number of species to be discussed is
rather high, it is proposed in the first instance to give a generalized description of the
main features in which they all agree, illustrating this fully with reference to a limited
number of sample types. This description will occupy the rest of this chapter, after
which the separate peculiarities of individual species, together with the evolutionary
analyses of all of them, will be added in the chapter which follows.
The choice oi Cyrtomium falcatum (Fig. 163) as the main illustrative sample type has
been dictated partly by historical reasons and partly for convenience. It was a fern
grown extensively for the market as an ornamental plant in the neighbourhood of
Manchester before the war, and unHmited supplies of material were made available
to me by the kindness of Messrs CHbran of Altrincham, who, without charge, gave me
free access to their nurseries. Although, as will be seen in the next chapter, I have since
160
APOGAMOUS FERNS. THE GENERAL PHENOMENON
supplemented this by several important samples of material of wild and botanic garden
origin, the normal strain of commerce was in the first instance my principal source of
information. Other species, notably Pteris cretica, at a later stage showed themselves to
be more amenable to cytological treatment and gave better preparations more easily;
yet others, e.g. Dryopteris Borreri and Phegopteris, were detected as relatively abundant
sources of material in the British flora. All these advantages have been utilized in the
evolutionary analyses, but the chief historical interest still lies with Cyrtomium. Besides
Fig. 163. Part of a frond oi Cyrtomium falcatum (L.f.) Presl from a pressed
greenhouse plant of commercial origin. Natural size.
being one of the first three apogamous ferns to be discovered (de Bary, 1878), the other
two being Pteris cretica and Dryopteris Filix-mas (or D. Borreri) var. cristata Hort., the
existence of Miss Allen's rather imperfect description of the cytology quoted above
makes it seem of greater scientific value to emend this rather than to utilize some
other species, such as Pteris cretica, about which no confusion is likely to arise.
Before starting the description, however, it may be well to anticipate what will
be said in the next chapter to the extent of explaining the change of nomenclature
(Christensen, 1930) which has been introduced since Miss Allen's time. The old species
' Aspidium falcatum', besides being relegated to a separate small genus Cyrtomium
composed of about a dozen species, has itself been split into three, each with a diflferent
geograpliical area in eastern Asia and Africa. Miss Allen's material was from Wisconsin
Botanic Garden and was probably C. falcatum proper, since this is the most commonly
MFC
161
II
APOGAMOUS FERNS. THE GENERAL PHENOMENON
grown. The other two species are C. Fortunei and C. caryotideum, both previously treated
as varieties. Their separation as species does not, as it turns out, seriously affect the
issue. I have had wild material of both C. Fortunei and C. caryotideum and botanic garden
material of C. falcatum from several sources including Wisconsin. All are cytologically
indistinguishable, although no doubt slightly different genetically, and therefore for
the present purpose they can be used interchangeably. Illustrations of the various types
of fronds will be found in the next chapter.
The early stages of sporangial development in all Polypodiaceous ferns, whether
apogamous or sexual, are identical, and consist of a limited but very precise set of
f
f
\ A' d
Fig. 164. Section of part o{ a sorus oi Cyrtomiumf alcatum {h.f.) Presl
to show various stages of young sporangia, x 250.
cleavages in what was originally a single superficial cell. After cutting off the stalk,
which soon becomes a short filament, the terminal cell undergoes four oblique cleavages
which separate the sporangium wall from a central tetrahedral cell (Fig. 1 65 a) . From
this central cell (which appears triangular in all types of section) a further set of cleavages
parallel to the four sides separates the tapetum from the archesporium (Fig. 165^,
centre). The tapetum gives rise to two layers of cells which are nutritive in function
(Fig. 16^ b, left), and the archesporium undergoes a sequence of four synchronized
mitoses (cf Fig. 166) to give a central mass of cells which, in the sexual species, are
ultimately sixteen in number. These sixteen cells then enlarge considerably and begin
to round up to become mother cells, each of which will give rise to four spores after
undergoing meiosis. During the rounding up which accompanies the early meiotic
prophases the mother cells tend to separate somewhat from each other, and the inter-
stices between them become filled with protoplasm from the inner tapetal layer which
becomes plasmodial. This is visible in many of the photographs of meiosis in section
included in previous chapters. The walls of the inner tapetal cells finally disappear,
162
APOGAMOUS FERNS. THE GENERAL PHENOMENON
and protoplasm and nuclei from this layer closely invest the mass of mother cells until
the spores are ripe. At the same time the sporangium enlarges considerably owing to
increase of its layer of wall cells, so that during the act of meiosis the mass of tapetum and
mother cells appears as if suspended in a cavity far too large for it and which will not
be filled until the spores themselves reach their final size. Some of these stages can be
seen in Fig. 164 and elsewhere in this chapter and the next.
Fig. 165. Sections to show very young stages of sporangial development
in Cyrtomium falcatum (L.f.) Presl. x 500. For description see text.
In the apogamous ferns extreme uniformity prevails over the early stages up to the
first few synchronized mitoses of the archesporium (Figs. 166, 167 a) after which one
of four dififerent things may happen.
(i) In some sporangia all four successive archesporial cleavages may be completed
and sixteen spore mother cells result (Fig. 170a). The proportion of such sporangia
varies somewhat from species to species; in some they are abundant and in others
extremely rare though they never seem to be entirely absent. Sporangia of this type are
of extreme interest from an evolutionary point of view for they display the true pairing
homologies of the chromosomes. In most cases, however, they are of no importance for
the reproduction of the species and their spores abort.
(2) The second type of sporangium is the one which is responsible for reproduction.
In this the first three archesporial cleavages are normal, but the last division which
163 11-2
APOGAMOUS FERNS. THE GENERAL PHENOMENON
should change the eight-celled archesporium into the sixteen-celled is imperfect.
Metaphase starts in each of the eight archesporial cells in the usual way, the split chromo-
somes taking their places on the spindle (Fig. 167 a) ; but there the division ends. There
is no anaphase separation of half-chromosomes and cleavage of the cytoplasm is also
omitted. The mass of split chromosomes remains in the centre of the cell losing some-
thing of its regular outline (Fig. 167^) and then reverts to the resting state (Fig. 167 c).
As may be seen by comparison of Fig. 166 b with Fig. 167 c before and after this abnormal
division, the number of cells present remains unaltered, four of the eight being generally
contained in any one median section no matter what the plane of cutting. The nuclei
■♦^^^r
n
m
Z^*!'!!
%
^
%
-SlSi"
^^'>
:-^
Fig. 166. Young sporangia of C>r;omm?n/a/ca<Mm (L.f.) Presl from sections, x 1000. Showing two stages
in the formation of the eight-celled archesporium, (a) the younger, (b) the older.
of the later stage are, however, distinctly larger and their shape at first less regular.
In due course they become mother cells (Fig. iGyd). Meiosis is then exceedingly regular
(Fig. 168), every chromosome pairing with its sister half, and thirty-two large well-
filled spores result. An accurate chromosome count made during meiosis will, however,
at once show what has occurred. The number of pairs which present themselves at
metaphase of the first meiotic division or at diakinesis is not half that of the single
chromosomes in a root or other somatic cell of the parent plant but identical with it. A
demonstration of this has already been given for Phegopteris (Figs. 69 and 70, Chapter 5),
and it is one of the ways in which apogamy can be detected before the spores are sown.
The explanation is that the abnormal premeiotic mitosis has momentarily doubled
the chromosomes present and therefore meiosis merely restores the condition to
164
Fig. 167. Cyrtomium falcatum (L.f.) Presl, the abnormal mitosis in the eight-celled archesporium. In each
case only four cells are visible. From sections stained with haematoxylin and Bismark brown, x 1000.
a. The premeiotic metaphase still normal, b. The premeiotic 'anaphase' with the chromosome still
on the equatorial plate, c. The premeiotic telophase. Restitution nuclei have been formed in each
cell without cytoplasmic division, d. The beginning of meiosis. Eight (four visible) giant mother
cells each with twice the previous number of chromosomes. Note signs of cytoplasmic cleavaere in
one of them.
APOGAMOUS FERNS. THE GENERAL PHENOMENON
that characteristic of the rest of the sporophyte, but the haploid state is not thereby
attained.
(3) The third type of sporangium is a variant on that just described and is in a sense
an imperfect version of it. The cytoplasmic activities which normally result in cell
cleavage may not be entirely suppressed but may be present in unco-ordinated forms
which affect in a very striking way the shapes and behaviour of the resulting mother
cells. Sporangia of this kind are those which were interpreted as stages of nuclear migra-
tion and fusion when first seen. A superficial resemblance to such a process they un-
doubtedly possess. The nuclei, after loss of the spindle, may become irregularly lobed
(Fig. i6ga, b and d), cell walls partially crossing the cell may be laid down, often in
relation to such lobes (cf. Fig. 169 a, b), and sometimes complete cleavage into two
unequal portions containing different-sized pieces of the restitution nucleus may result
3^* m
\
yr ■**#
Fig. 168. Meiosis in Cyrtomium falcatum (L.f.) Presl from sections, x 1000.
a. Diakinesis. b. The first metaphase.
(Fig. 169 c,/, g). That the cleavage, in the last event, involves a passive amitotic con-
striction of a restitution nucleus and not a mere inequality of anaphase distribution of
normally separating half-chromosomes is proved by the complete regularity of chromo-
some pairing no matter how large or small a piece of nucleus may have been con-
stricted off; this could not be attained by any process of random distribution of
non-homologous half-chromosomes but must denote the random separation of groups
of split chromosomes with their halves still in close contact. As may be seen from
several of the figures quoted, which refer to a number of different species, not all
the mother cells in a sporangium may be affected in this way, but only one or a few.
Since the distribution of chromosomes to the constricted portions is certainly at
random the nuclei so formed can hardly fail to be genetically unbalanced, and
abortion of the resulting spores is therefore virtually certain.
(4) The fourth type of sporangium was not observed by Dopp, but I have come across
it from time to time in almost every species. Occasionally the abnormality affecting the
premeiotic division in the eight-celled sporangia may occur twice running and affect the
four-celled stage also. In such cases only four giant mother cells are to be found at
166
*jr
Ull-tH
aB*»8g.^if ■'%=ji»..
.w
¥
'iN^
*.i^:--/
1 ■»»> ^
"'•I fc%
%
#
Fig. 169. Various examples of imperfect cleavage resulting from the abnormal premeiotic division
( = developmental type 3, p. 166) from sections, x 1000. a. Dryopteris atrata (Wall.) Ching [Nephro-
dium hirtipes (Bl.) Hook. ) showing partial cleavage of one cell. b. The same in Dryopteris remota A.Br.
at early meiotic prophase, c. The same in Pteris cretica L. var. albolineata. A small cell has been
cut off from a large one, but both are now in an early state of meiosis. d. Dryopteris Borreri var.
polydactyla Wills, e. Tripolar meiotic spindle in Pteris cretica var. albolineata. f, g. Dryopteris Borreri
var. polydactyla Dadds. Two very unequal-sized meiotic metaphase plates in the same sporangium.
167
APOGAMOUS FERNS. THE GENERAL PHENOMENON
meiosis, each with four times instead of twice the somatic chromosome number (cf.
Fig. 69 b, Chapter 5) . In spite of their high chromosome number such mother cells are
quite normal in their subsequent behaviour. It might reasonably be expected that
quadrivalents would be formed, since each chromosome is certainly present in quad-
rupUcate and all four homologues lie close together since they are quarters of the same
.*>
f
s
^ m S
mm
%
•^ t .;^ ■
I
// "% I
M
'5^'
i '^--^
0'
• »
1
t
'0
c
d
Fig. 170. Different types of sporangia in a single sorus of Cyrtomium Fortunei J.Sm. from sections.
a. Sixteen-celled sporangium, x 500. b. Eight-celled sporangium, x 500. c. Four-celled spor-
angium. X 500. d. The second abnormal mitosis in a potential four-celled sporangium by means
of which the chromosome number is quadrupled, x 1000.
original chromosome (Fig. i^od). This, however, does not occur. Pairing seems to be
confined to sister chromosomes only, and the result is a meiosis which is as undistorted
in all details as is that of the eight-celled sporangia. Such sporangia when ripe contain
only sixteen giant spores, each with twice the nuclear content of the plant which bears
them. Their subsequent fate is unknown, but it is highly probable that they may be
responsible for some of the cases of polyploidy by simple chromosome doubling which
have been met with in apogamous ferns, notably in Pteris cretica (see next chapter).
168
APOGAMOUS FERNS. THE GENERAL PHENOMENON
Figs. 1 70 and 1 7 1 summarize some of these facts for one particular sorus of Cyrtomium
Fortunei in which sixteen-celled (Fig. 170a), eight-celled (Fig. 170^) and four-celled
sporangia (Fig. 170c) were found in full meiosis simultaneously. This is somewhat
unusual, although four-celled sporangia are sufficiently common to be hsted as a separate
type. Their frequency almost certainly varies with the genetical condition of the plant,
but only once have they been seen in such relative abundance as to dominate the repro-
ductive picture. This was in a monstrous form of Dryopteris Borreri, almost certainly a
descendant of a hybrid between that species and one of its sexual relatives; but un-
fortunately nothing further is known about this plant except a dried frond and some
cytological preparations.
It is clear that diversity of this degree between the developmental histories of the
different sporangia in a sorus is a matter of great cytological, genetical and physiological
^^4$: *%ill* '
a
Fig. 171. Details of chromosome pairing in two of the sporangia of Fig. 170, from sections, x 1500.
a A sixteen-celled sporangium showing irregular pairing, b. An eight-celled sporangium showing
very regular pairing, since the chromosome number in this has been doubled by the abnormal
premeiotic division.
interest. Much further work is likely to be necessary before the causal mechanism will
be understood, but a few further facts of a genetical kind will be forthcoming at the end
of the next chapter, while others of the many purely cytological points of interest must
be left aside for consideration elsewhere, since they are irrelevant to the evolutionary
study which is the main purpose of this book.
Even in this Hmited context, however, the situation revealed is of unusual interest.
In the life history of the apogamous ferns a very remarkable compensating process is
present which is curiously difficult to equate with anything of a comparable nature in
other groups of plants or animals. It is, moreover, a complicated process in which
purely cytological aberrations in the sporangia must occur in the same organism as the
morphological aberrations in the prothalU which result in apogamy, or the continued
existence of the species would be impossible. That the somatic organization of the
leptosporangiate ferns lends itself rather easily to these aberrations is shown by the
relative frequency with which identical behaviour is found in isolated examples from
quite unrelated genera. That the whole abnormality must have arisen in each case
suddenly seems almost inescapable, since it is difficult to visualize any mechanism
169
APOGAMOUS FERNS. THE GENERAL PHENOMENON
which would at the same time be reproductively effective for its development by stages.
For similar reasons a simple qualitative mutational mechanism seems unlikely. We
seem rather to be dealing with the effects of a generalized disturbance of a quantitative
kind involving several processes. We are ignorant of the chemical nature of these
processes, though some further information on the mode of origin of a generalized
disturbance will emerge from the next chapter.
Another point of interest to comment upon in passing is the surprising variety of
sporangial life histories which all these ferns possess and to note its evolutionary con-
sequences. If any of the spores produced by the types of sporangia described as (i),
(3) and (4) on pp. 163, 166 above should prove viable, a considerable saltation in
morphology and genetical constitution might result. That under certain circumstances
changes of this kind do, in fact, occur, will also be shown in the next chapter.
SUMMARY
A general description of the sporangial development found in all known cases of
apogamous ferns is given, with photographic illustrations principally selected from
Cyrtomium ( = Aspidium) falcatum sens. lat. There are four main types of sporangia, all
of which may be found together in one sorus. These may be designated according to
the number of spore mother cells undergoing meiosis as sixteen-celled, eight-celled,
eight-celled with partial cleavage and four-celled. Only the eight-celled sporangia are
normally effective in reproducing the species and they give rise to spores with the un-
reduced chromosome number.
170
CHAPTER 11
APOGAMOUS FERNS [cont,]
EVOLUTION OF THE SEPARATE SPECIES
The nature and possible mode of origin of the various apogamous species listed on
p. 158 may next claim our attention. And here the reader should perhaps be warned to
expect a rather extreme measure of compression in the description of each case as it
arises, for only so can the results of an extended inquiry be brought within the compass
of a single chapter. Each species must be analysed on its own evidence without a pre-
conceived interpretation. But in the concise presentation of evidence it will scarcely be
possible to convey anything of the charm of the plants themselves or of the many details
of interest in their structure, behaviour or distribution which reward the investigator
for the labour involved in piecing the evidence together. In the colourless description
of species after species the impression of repetition and sameness may indeed pre-
dominate. This, it should be remarked, is, in a sense, what we are after. The differences
between species give personality to them, but only the resemblances are likely to show us
anything of the general principles which they all share, and the principles which we
may hope to discern in this context are those which underlie the development of
apogamy in ferns as a whole.
Sources of information about the evolution and origin of an apogamous species are
of three, or perhaps four, kinds. The chromosome number alone may be highly informa-
tive if enough is known about the cytology of related sexual species; in the absence of
such knowledge chromosome number by itself is, of course, uninformalive. Of greater
value is observation of the mode of pairing of the chromosomes in the sixteen-celled
sporangia (type (i), p. 163 above). This can give a very great deal of information
regarding the cytogenetical make-up of the plant involved, even in the complete
absence of related species for comparison. Thirdly, significant differences can some-
times be detected in the relative frequency with which the various types of sporangia
appear. Observations of this kind are sometimes of value (notably in Dryopteris Borreri)
for confirming cases of suspected hybrids between an apogamous and a sexual species.
Lastly, as was shown by Dopp (1939) it is sometimes possible to synthesize hybrids
between apogamous and sexual species, and in such cases observation of chromosome
pairing should be as informative as in other cases of hybrids of known parentage.
Owing to the labour involved, very little work has yet been done using this last method,
but the first two, and to a less extent the third, have given information of considerable
value about almost every species.
It will be convenient to start this chapter with Pteris cretica, which will serve both to
round off de Bary's account previously quoted and to supply us with a species more
amenable than most to cytological treatment and of interest as representing a genus not
of the Dryopteroid affinity.
171
Fig. 172. Living sterile frond of Pteris cretica L. var. major Hort. Natural size.
172
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
Pteris cretica L. is widespread and abundant in the tropics of the Old World, although,
somewhat surprisingly, it has a number of outlying stations in Europe and, owing to its
ease of cultivation and tolerance of the dry air of dweUing houses, it is an even greater
favourite as an ornamental plant of commerce (cf. Fig. 172) than is Cyrtomium. Most
of the considerable range of horticultural varieties have arisen in cultivation, and it is
doubtful whether the genuine wild species is present at all in greenhouses at the present
Fig. 173, Meiosis in Pteris cretica L. from sections stained in haematoxylin and counter-stained with
Bismark brown to show unusual excellence of fixation, a. Diploid P. cretica first meiotic metaphase
in an eight-celled sporangium, x 1000. b. The same, anaphase, c. The same in a sixteen-celled
sporangium. Note smaller cells and lagging chromosomes at both metaphase and anaphase.
d. Triploid Pteris cretica var. albolineata. Diakinesis in an eight-celled sporangium, x 750.
time. An exception is, perhaps, var. albolineata, for this was described by Hooker as a
wild variety with variegated fronds native to the Far East, and comparison which
I have been able to make with a reputedly wild specimen from Ceylon supplied by
Peradenya Botanic Gardens showed no detectable difference of any kind between it
and examples of the variety already in cultivation in England. It has, however, long
been known (cf de Litardiere, 1920) that the native European species has fewer
chromosomes than the morphologically normal 'var. major' of commerce. The numbers
given by de Litardiere were 2/2 = 60 and 120 respectively, the low number being
173
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
obtained from Italy and from Corsica. In my experience these numbers are very nearly
correct (see Fig. 174), and the wild European species may thus be regarded as diploid
in contrast to 'var. major' which is tetraploid. All other commercial strains that
I have investigated, notably var. Wimsetti and var. Childsii, are also tetraploids which
is the reason for thinking that the wild type had fallen out of use.
The principal material of Pteris cretica used by me has been the following:
(i) The horticultural strains 'var. major' and 'var. Wimsetti' grown for the market,
together with Cyrtomium falcatum, by Messrs Clibran of Altrincham, to whom I am
extremely grateful for unrestricted access to their greenhouses.
P. crehca n ^ 58
Fig. 174. Meiosis in diploid Pfem cr^^ica Fig. 175. Explanatory diagram to
L. permanent acetocarmine, from an Fig. 174. x 1500. .
eight-celled sporangium to show de-
tailed chromosome count ('h' = 58).
X 1000.
(2) var. albolineata from Kew and Ceylon together with var. albolineata-cristata, a
crested form of albolineata otherwise closely resembhng it, from Kew.
(3) Wild European material collected by myself from a locality near the western
shore of Lake Maggiore in 1937 and since maintained in cultivation.
(4) Wild tropical material brought alive to Kew from Uganda in 1938 but un-
fortunately lost as a result of the war.
Although the horticultural varieties first mentioned (i) were the earliest to be in-
vestigated and from them the general oudine of the behaviour obtained, in the account
which follows attention will be almost confined to the three wild samples listed under
(2), (3) and (4), owing both to the obvious advantages of wild over horticultural
material but also to the spread of chromosome numbers displayed, which recalls in
some ways what has already been noted in Dryopteris Borreri.
Chromosome counts of all these strains showed that whereas the ItaHan material, as
expected, was of a low chromosome number with 2n = c. 60 (actually 58, see Figs. 174,
175), which may be interpreted as diploid, the Uganda material was tetraploid with
2n ^c. 120 (cf Fig. 176^). It is of some interest to have traced a tetraploid to a known
geographical locality. On the other hand, 'var. albolineata' is triploid with yi = c. 90
(Fig. 176c), and so is 'var. albolineata cristata' .
All forms of Pteris cretica give an exquisite quality of fixation readily, as may perhaps
already have been seen in Fig. 173 and others. It is also fortunate that sixteen-celled
174
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
sporangia are more abundant than in Cyrtomium falcatum, and details of chromosome
pairing in such sporangia are therefore available for all the polyploid types. The
greatest interest naturally attaches to such pairing in the diploid which in this case is
hkely to be the oldest type, a view with which its apparently rehct and disjunct
occurrence in Europe is in full accord.
The first impression given by chromosome pairing in the sixteen-celled sporangia of
Pteris cretica is its irregularity. Pairs are numerous and absorb the majority of the
Fig. 176. Meiosis in the polyploid series of Pteris cretica L. from sections, x 1000. a. First meiotic
metaphase in an eight-celled sporangium of the diploid {2n - 58). b. The same in a sixteen-celled
sporangium, c. Triploid var. albolineata {^n = c. 90) an eight-celled sporangium, d. The same in
a sixteen-celled sporangium, e. The wild tetraploid from Uganda (4« = c. 120) an eight-celled
sporangium. /. The same in a sixteen-celled sporangium.
chromosomes, but there are also some trivalents together with a few probable quadri-
valents and a residue of unpaired chromosomes remains. Various views are shown in
Fig. 177^ and c.
In contrast to this, the sixteen-celled sporangia of triploid and tetraploid show fewer
unpaired chromosomes and more multivalents involving larger numbers of chromo-
somes. Some of these, as found in the Uganda tetraploid, are shown in Fig. 178, in
which a very large multivalent group is seen at diakinesis. Another expression of the
same thing is the reduced number of lagging univalents visible in side views of meta-
phase; this is perhaps detectable by comparing the bottom row of Fig. 176 in which
this stage in diploid, triploid and tetraploid are placed side by side.
With regard to the interpretation of these observations, the presence of multivalents
in tetraploid P. cretica need not surprise us, since some are also to be found in the diploid.
175
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
Since a mechanism for obtaining tetraploids from diploids is known to exist in the four-
celled sporangia, there is no reason to regard the Uganda plant as anything other than
a derivative by simple chromosome doubhng from the simpler state still retained in
Fig. 177. Details of chromosome pairing in diploid Ptem cre^zVa L. from sections, x 2000. a. Diakinesis
in an eight-celled sporangium with very regular pairing, b. Diakinesis in a sixteen-celled spor-
angium showing univalents and multivalents, c. The same at first metaphase showing a univalent
a trivalent and other groups.
Fig. 178. Pairing at diakinesis in a sixteen-celled sporangium of tetraploid Pteris cretica L.
from Uganda showing complex multivalents.
the European form of the species. With regard to the triploid we are in the same
dilemma as in triploid Dryopteris Borreri, already to some extent discussed in a previous
chapter. A hybrid between apogamous Pteris cretica and some related sexual species
seems the most probable explanation.
With regard to diploid P. cretica there are perhaps two alternatives. The first and
most probable is that it is a hybrid between two related species with much though
not complete homology between the chromosomes of the two gametic sets. Another
176
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
possibility must, however, be recognized. The wide geographical range and apparently
relict status of P. cretica in Europe must
denote a high antiquity for that species
during which time chromosome changes
(e.g. translocations and the like) may have
taken place, and in the absence of natural
selection their effects may have accumu-
lated. It is conceivable that all the
irregularity of pairing observed both in
production of multivalents and of univa-
lents may actually have arisen in this way
after the apogamous habit was established.
This would mean, if true, that for this one
species the possibility exists that apogamy
itself could perhaps have arisen in a
formerly sexual species, not by an act of
hybridization, but by a process of internal
differentiation of a genetical kind. This
explanation is not the most likely one,
for it would be expected that traces of a
sexual form of similar morphology would
be found if mere mutation had produced
the apogamy, and there is so far no in-
dication that such a sexual form exists.
It is, nevertheless, perhaps of importance
to raise this alternative explicitly at this
point, for, as will shortly be seen, it is an
explanation which on cytological grounds
is virtually excluded for every other apo-
gamous species analysed.
It may now be suitable to complete the
account o{ Cyrtomium. As already explained
(Chapter lo) there are now three species
instead of one to consider owing to the
splitting of the old aggregate ' Aspidium fal-
catum' into three microspecies now known
as Cyrtomium falcatum Presl, C. Fortunei
J.Sm. and C. caryotideum Presl. The first
has already been illustrated in Fig. 163,
the second is represented in Fig. 180 and
the third in Fig. 1 79. All three have been
Fig. 179. Cyrtomium caryotideum {y^a\\.)Vvc%\
from Uganda. A young, live frond in
cultivation. Natural size.
Mpc j"yY 12
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
in cultivation for many years in Europe and America, both in botanic gardens and
as ornamental plants of commerce.
Owing to the rather wide discrepancy between my results and those of Miss Allen
(1914) with regard to the chromosome numbers, it was necessary to examine a wide
sample of the various species with rather pedantic attention, since a superficial com-
parison with Pteris cretica might easily have led to erroneous conclusions. The material
available to me was as follows :
(i) Horticultural plants oi Cyrtomium falcatum, grown for the market in the nurseries
of Messrs Clibran of Altrincham, Cheshire.
(2) C. falcatum var. Rockfordii Hort. in the Royal Botanic Gardens, Kew.
Fig. 180. Q'r/omzMm For/wnf 2 J. Sm., part of a dried cultivated frond. Natural size.
(3) C. falcatum var. Rockfordii Hort., raised from spores obtained from the Botanic
Garden, Wisconsin, U.S.A., and thought to be identical with Miss Allen's material.
(4) C. Fortunei of unknown origin, long established at Manchester University Experi-
mental Ground.
(5) C. Fortunei, wild from the neighbourhood of Pekin, raised from spores sent by
Dr C. Ching.
(6) C. caryotideum, wild from Uganda, conveyed alive in 1938 to Kew, together with
the Pteris cretica previously referred to, but, like that species, subsequently lost owing to
enemy action during the war.
From this rather extensive range of specimens the greatest scientific interest naturally
attaches to the wild specimens (numbers (5) and (6)) and to the Wisconsin material
(number (3)). None of these was, however, obtained until the work was far advanced,
and in the general account already given in Chapter 10, numbers (i) and (4) were
principally quoted. Enough has been seen of the others, however, to make it quite
certain that they are not different in any way which can be detected cytologically.
Evidence on the chromosome numbers of the three species of Cyrtomium is contained
in Figs. 1 81-184. In sections (Figs. 181, 182), only approximate counts can be made
which, in each of the three cells figured, place the number as 'not less than 119 nor
more than 123'. The cell of Fig. 181 a and 182 a, it should perhaps be pointed out, is
from Wisconsin. Only with squashes can greater precision be reached, and in each of
178
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
the cells shown in Fig. 1 83 a and b, which relate to C. falcatum and the wild C. Fortunei
from China respectively, the number is 'not less than 122 nor more than 123'. In the
cell of Fig. 183 c of wild C. caryotideum from Uganda the number is exactly 123.
The significance of this chromosome number is considerable. It is about twice that
recorded by Allen, but it is not a tetraploid and there seems httle doubt, especially in
view of the results obtained by reinvestigating Wisconsin material (Figs. i8ia, 182 a),
Fig. 181. Cyrtomium chromosome counts from section, a, b, c. Three different focal levels of a plate
of chromosomes in a root of the Wisconsin plant (see text), x 1500. For explanatory diagram see
Fig. 182a. d. Meiosis in C. falcatum (L.f.) Presl, plate of chromosomes slightly dismembered by
pressure, x 2000. For explanatory diagram see Fig. 1826. e. Mitosis in a root of C. /a/catom var.
Rockfordii from Kew. x 1000. For explanatory diagram see Fig. 182 c.
that Miss Allen's estimate must have been due to technical error and not to genetical
diiTerence of her specimen.
The obvious interpretation of the actual number found is that it is that of a triploid,
since the monoploid number 41 is known to characterize a number of very closely
related genera, notably Dryopteris and Polystichum. This interpretation is to some extent
confirmed by the evidence from chromosome pairing in the sixteen-celled sporangia.
Trivalents are certainly present (Fig. 185), together with pairs and a large number of
univalents. Trivalents are not in themselves diagnostic of triploidy if present in small
numbers, and if the subject is not an autotriploid, nevertheless, they are some confirma-
tion of the status of the parent plant if this had been diagnosed from other evidence.
179
12-2
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
In. addition, the general appearance of the cells in question, especially the relatively
large number of unpaired chromosomes, is very strong confirmation of a necessary
concomitant of triploidy (whether this be auto- or alio-), namely, hybridity. For the
attainment of the triploid state wherever found would seem to necessitate an immediate
or antecedent cross between two dissimilar plants, at least one of which must have
been sexual.
Since all three of the wild species and the one horticultural variety, var. Rockfordii,
have the same chromosome number, it must be further assumed that the morphological
i9l«#
^^^^
Cc/r/^om/am
3n = c. /20
Fig. 182. Explanatory diagrams to Fig. 181. All x 3000. a. The Wisconsin plant. Chromosomes in
focus in Fig. 181 a shown in black, those in focus in Fig. 181 c in dots, and the remainder in outline.
Approximate count is 1 19-123. b. The cell of Fig. 181^. Approximate count c. 121. c. The
cell of Fig. 181 «.
differences which distinguish them are secondary to their apogamous habit and have
been developed later than this by a process presumably of genie mutation. It is obvious
that in any apogamous species non-injurious mutations would tend to be transmitted
indefinitely without disturbance or segregation.
The next species to claim our attention can be Dryopteris atrata (Wallich) Ching (Fig.
186), formerly known as Nephrodium hirtipes and partially investigated by Steil (1915,
1919) in work which has been already quoted (p. 160 above). The species in the wild
state occurs from China to the Himalayas and is available only in botanic gardens,
since it is not a fern of commerce. My material was obtained from Kew and since
180
•
Fig. 183. Squash prepara-
tions of meiosis in Cyrto-
mium. X 1000. a. C. Jal-
catvm (L.f.) Presl in bal-
sam after acetocarmine.
b. C. Fortunei J.Sm., the
wild plant from China, in
liquid acetocarmine. c. C.
caryotideum (Wall.) Presl
in balsam after aceto-
carmine. For explanatory
diagram see Fig. 184.
<*
^i^
w ^
Cj/rZ-om/'um "n" = /^J ^%
Fig. 184. Explanatory diagram to Fig. 183c:. x 150c. 'n'= 123.
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
Steil's was also of botanic garden origin it is possible that the two are identical. This
supposition is not affected by the change of name, which is quite recent* (Ching, 1933).
a be
Fig. 185. Details of chromosome pairing in sixteen-celled sporangia oi Cyrtomium Fortunei J.Sm., from
a section, x 3000. a. Pachytene showing a trivalent. b. Diakinesis showing trivalents. c. Meta-
phase showing a trivalent, a univalent and other groups.
Fig. 186. Dryopteris atrata (Wallich) Ching. Part of a dried frond
of the plant used. Natural size.
Some of the sporangial peculiarities of Dryopteris atrata have already been mentioned
in a previous chapter, and it only remains to add such details as may be of evolutionary
importance. The most significant detail is again the chromosome number. As in the
case of Cyrtomium I differ considerably from previous investigators, in this case Steil
(19 19) and Andersson and Gairdner (1930), for reasons which I can only interpret as
* The change of name results from a splitting of the former collective species Nephrodium hirtipes into
two, the Chinese portion now constituting Dryopteris atrata and the Indian portion retaining the epithet
hirtipes. On comparison with Ching's diagnoses (1933), the botanic garden form, the origin of which is
unknown, was found to conform to the Chinese variant. The labels were emended in consequence, though
the plant is the same as that formerly disseminated under the older name.
182
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
technical deficiency in the earlier workers. As determined by me (Figs. 187-190) the
chromosome number of Drjopteris atrata is indistinguishable from that of Cyrtomium
and is of the order of 120 on the imperfect evidence of sections and exactly 123 when
squash methods are available.
Chromosome pairing in the sixteen-celled sporangia of Dryopteris atrata is shown in
^^ Fig. 189 a. Unlike the case of Cyrtomium, pairs
jJHfc are few and multivalents apparently absent; the
"^^^ resemblance is therefore with Dryopteris remota
jtft^^ ^»^ described by Dopp. Since from the chromosome
^^^j^P-^^^^ - t***L number and systematic position of the species it
^ ^■^■1^ "^ may safely be assumed to be another triploid,
the absence of homology among the chromosomes
^'J^^
D. o/ra/-^ 3/7 = c. /20
Fig. 188. Explanatoiy diagram to
Fig. 187a. X 3000.
Fig. 187. Dryopteris atrata (Wallich) Ching. Mitotic chromosome counts from section:, a, A root,
showing a good metaphase plate surrounded by heavily staining cytoplasmic inclusions, x 1000.
For explanatory diagram see Fig. 188. b. A dividing cell in a young sporangium, x 1500.
present would seem in this case, for the first time in this chapter, to rule out the possi-
bility of really close affinity between the two parents. Autopolyploidy followed by minor
evolutionary changes cannot be wholly excluded from either Pteris cretica or Cyrtomium,
but in Dryopteris atrata there is no reason of any kind to bring it to mind. The species
here seems to be a triploid hybrid, either formed directly or by descent from some other
polyploid in which there is virtually no affinity between the chromosomes of the
hybridising parents. In this case it is therefore necessary to accept an interspecific and
not an intervarietal cross in its immediate ancestry. [For another similar case see foot-
note on Asplenium monanthes L. added in proof on p. 195.]
Phegopteris, the Beech Fern, has already been dealt with at some length in Chapter 5,
and there is nothing to add to that description. The sixteen-celled sporangia in this
183
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
species are unfortunately so rare that I have not yet seen this type of meiosis, in spite of
annual fixations made for the purpose over many years. The possible origin of the
species is therefore at present unknown, since the chromosome number (90) is unlike
.- ... '•t.iskMfa
Fig. 189. Meiosis in Dryopteris atrata (Wallich) Ching. a. Two mother cells surrounded by tapetum
from a sixteen-celled sporangium in a section showing irregular meiotic figures composed largely
of unpaired chromosomes, x 1000. b. Spread metaphase from an eight-celled sporangium in
balsam after acetocarmine. x 1500. For explanatory diagram see Fig. 190. 'n'-c. 122 =
probably 123.
P^l ^*
D. atra/^a
'/?"= /^3
Fig. 190. Explanatory diagram to Fig. 189^). x 2000.
that of any of the other genera with which Phegopteris has from time to time been classed
and there is therefore as yet no clue to its nearest affinities.
Somewhat similar uncertainty hangs over the genus Pellaea, of which one species,
P. atropurpurea (L.) Link, has been available to me from spores collected wild in Cali-
184
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
fornia and communicated to me by the kindness of Mr Alston of the British Museum.
In this case the internal facts are easily ascertained, but their interpretation is hampered
by the absence of any other information re-
garding related species. The form of the leaf
of the specimen used is shown in Fig. 191,
and the chromosomes in a root, an eight-
celled sporangium and a sixteen-celled sporan-
gium, are shown in Figs. 192 and 193. The
chromosome number is approximately (and
perhaps exactly) 87, and pairing in the
sixteen-celled sporangia falls into approxi-
mately equal numbers of pairs and univalents.
Both this pairing and the chromosome number
itself are strongly suggestive of another triploid,
although acceptance of this interpretation
requires some independent evidence that a
monoploid complement of 29 exists in this
group. If it does the pairing could be regarded
as that of a backcross between an allotetraploid
and one of its diploid parents, one at least of
which must have reproduced sexually. The
search for sexual species of Pellaea or related
genera * with the required chromosome num-
bers may therefore be recommended to local
botanists to whom these plants may be acces-
sible in the wild.
Returning now to Dryopteris we have D. re-
mota A.Br, and D. Borreri Newm. still to
consider. With regard to D. remota much
has already been said in Chapter 5, and the
chief thing to recall is the hope that it will
shortly be synthesized. Whether or not the
parentage deduced for it on p. 79 is correct,
however, the evidence from the sixteen-celled
sporangia shows clearly that, as in D. atrata,
there is almost complete lack of homology
between the chromosomes of the two com-
ponent species. As Fig. 194^ displays, the
almost complete failure of pairing of all the
chromosomes in the sixteen-celled sporangia of this species, in marked contrast to the
regularity of pairing in the eight-celled sporangia, is as characteristic a feature of the Irish
specimen, as it appears, from Dopp's description (1932), to be of continental material.
* The existence of « = 29 in the related genus Pteris has, of course, actually been demonstrated in
this chapter by finding that diploid Pteris cretica has 2n = 58 (p. 174 above).
185
Fig. igi. Pellaea atropurpurea (L.) Link, live
frond of a young plant gVown in cultivation.
Natural size.
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
The last of the apogamous species, D. Borreri, remains, and here we have richer
material than in any other recorded case, since the species is not only widespread and
abundant in our own flora but, unhke the Beech Fern which is also abundant but more
I
%y.
m
« ^ c
Fig. 192. Cytology of Pellaea atropurpurea (L.) Link, a. Two focal levels of mitosis in a root from
a section stained in gentian violet, x 1500. b. Meiosis in an eight-celled sporangium in balsam
after acetocarmine. x 1000. c. Meiosis in a sixteen-celled sporangium showing pairs and
univalents in balsam after acetocarmine. x 1000. For explanatory diagrams see Fig. 193.
\
•♦
Pef/aea "en" --87 n--87
Fig. 193. Explanatory diagrams to Fig. 192 a and h. x 1500.
uniform, we have a considerable variety of different forms to examine from which some
genetical evidence can be obtained even in the present state of the inquiry.
Two types of material should, in the first instance, be called to mind. On the one
hand, there are the host of monstrosities beloved of fern collectors of which the var.
polydactyla Wills and the var. polydactyla Dadds may be taken as samples, and on the
186
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
Other there is the unmodified natural wild species to which reference has already been
made in Chapter 5, with its range of polyploid chromosome numbers. Taking the
monstrosities first, the polydactylas need not detain us long. Evidence has already been
presented, both by Dopp and in Figs. 45-7 of Chapter 4, that these two forms follow
the normal story common to apogamous ferns in general, and it was already known to
.**»
••*» /
/// >
* ^
1 '.
" f
* ' V .
* "H < '
'*•' Jk oy«
■^ ^
• »^ •
1 If
''-I
• * •
, •.^- •
#1 « - .""«>
> •^«.
-4 ^«-, .
■ ^ ■/ •'
1
Fig. 194. Meiosis in Dryopteris remota A.Br, in balsam after acetocarmine. x 1000. a. From an eight-
celled sporangium with regular pairing, b. From a sixteen-celled sporangium with virtually no
pairing.
Fig. 195. One pinna oi Dryopteris Borreri var. polydactyla Dadds, Natural size.
Compare with var. Wills on p. 58.
Farmer and Digby (1907) that two different chromosome numbers are represented.
It is not surprising that Farmer and Digby 's actual counts are incorrect, considering
the early date of their investigation, and it is perhaps sufficient to emend their statement
by the demonstration already given (Fig. 45) that var. polydactyla Wills is a diploid
with 82 chromosomes in both generations, whilst var. polydactyla Dadds (Fig. 196) is
approximately triploid. Further, as Fig. 196^ illustrates, there is complete failure of
pairing in the sixteen-celled sporangia, at least in the latter variety.
Of far greater importance than the monstrosities which, at most, have a historic
interest, is the study of the naturally occurring wild species. Here we have evidence
from Britain, Switzerland, Germany, and more recently Norway. Fig. 197 indicates
187
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
the approximate distribution of the species in Europe as compiled in a recent study by
Nordhagen (1947).
^,^ ' ■ X.
***■ ■
'*! ^v^»...
4»
*
» -^
* %
-■ , f > '«t.^
i
-r. ». 4
«^-
, a b
Fig. 196. Meiosis in Dryopteris Borreri var. polydactyla Dadds in balsam after acetocarmine. x 1000.
a. From an eight-celled sporangium with regular pairing, b. From a sixteen-celled sporangium
with virtually no pairing.
Fig. 197. Map of the northern limits oi Dryopteris Borreri Newm. in Europe.
After Nordhagen (1947).
With regard to chromosome numbers in D. Borreri it has already been shown in
Chapter 4 that a polyploid series is present in grades ranging from diploid to pentaploid,
though not all of the same constitution. We are still imperfectly informed about the
188
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
geographical details of members of the series, although some preliminary facts are
available from the fom- countries mentioned above. In an unpubhshed communica-
tion from Mrs G. Knaben of Oslo, which I am permitted to quote, the first chromosome
Fig. 198. Comparable pinnae of the polyploid series in Dryo^tem ^orrm Newm. Natural size.
a, diploid; b, triploid; c, tetraploid; d, pentaploid.
count for Norway is 'approximately 125'. In Germany we have Dopp's record of
approximately 130, and in Britain and Switzerland there are my own records of
'«' = 123. There seems httle doubt therefore that the triploid is the commonest form
189
. ^-hf
!>' *
4
^
r
1 •
»
1
Fig. 199. Meiosis in sixteen-celled sporangia oi Dryopteris Borreri Newm. from sections, x 1000.
a, diploid; b, triploid; c, tetraploid; d, pentaploid.
190
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
of the species over most of its range. Local populations of diploids * have, however,
been met with both in Britain and in Switzerland, the only two countries in which a
range of material has been studied. Tetraploids and pentaploids have so far only been
encountered in England and Ireland, though they will probably be detected elsewhere
if search is made for them. They differ from the diploids and triploids in being single
individuals and not local populations, and all those so far met with have suggested by
their appearance that they are hybrids between D. Borreri and D. Filix-mas. Such an
origin is in agreement with their observed chromosome numbers, since a cross between
D. Filix-mas and diploid D. Borreri would be tetraploid and between/). Filix-mas and
triploid D. Borreri would be pentaploid. That hybrids of this constitution can be syn-
thesized readily has already been shown by Dopp (1939), and the experiment has
been repeated by myself more recently though the resulting plants are still too young to
provide full evidence.
:n**
Fig. 200. Enlarged detail of chromosome pairing in sixteen-celled sporangia of Dryopieris Borreri Newm.
X 1000. a. Diploid with only univalents, b. Pentaploid with numerous pairs.
Samples of comparable pinnae from diploid, triploid, tetraploid and pentaploid
specimens of the various types referred to above are contained in Fig. 198. The close
resemblance of diploid and triploid will perhaps be recognized. It is perhaps remark-
able that no tetraploid or hexaploid populations with the pure D. Borreri morphology
have yet been encountered, although a mechanism for their production (i.e. from four-
celled sporangia, cf. Chapter 10) undoubtedly exists. The reason for this is uncertain,
and further search may still perhaps reveal them.
Chromosome pairing in the sixteen-celled sporangia of one example of each of the
four grades of polyploidy is shown in Fig. 199 and in greater detail in Fig. 200. In all
cases pairing is irregular though it is not equally so throughout. Pairs are numerous in
tetraploid and pentaploid, though not so complete even in the tetraploid as to suggest
* For anyone interested in acquiring material of such local diploid populations four, which are well
known to me, may be listed as: (i) The Kentmere Valley near Kendal, Westmorland, both wood and
scree populations. (2) A wood near Bangor, north Wales. (3) A wood near Dartmouth, Devon.
(4) The neighbourhood of Dublin, a very golden yellow type. (Note that the tawny appearance of the
Dublin diploids is not diagnostic of the diploid state in other places.)
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
an origin by simple chromosome doubling. Pairing in the diploid, on the other hand, is
virtually absent (see Fig. 200 a). In certain cases (e.g. var. polydactyla Dadds) a triploid
may also show complete failure of pairing, but in the more normal forms of the species
some pairs are present in triploids, suggesting that not all the triploids found wild are of
the same genetical nature; some may perhaps be primitive triploids (i.e. the ancestral
type of the species whatever that may have been), while others (e.g. polydactyla Dadds)
may be triploids of secondary origin such as could be formed by a cross between an
apogamous diploid and a related sexual diploid. Until more is known about the nature
%
:^'
c d
Fig. 20 1 . Spores of the polyploid series in Dryopteris Borreri Newm. from glycerine jelly mounts.
a, diploid; b, triploid; c, tetraploid; d, pentaploid.
X 1000.
of related sexual species this problem cannot be solved, and it is equally impossible to
state with assurance which of the two first members of the series (diploid or triploid)
is in fact the older, for while a derivation of the latter from the former can have occurred
along the fines just indicated, a reverse derivation is also conceivable. Should any of
the spores produced by the sixteen-celled sporangia be viable, we know enough about
the breeding behaviour of triploids (cf. Osmunda, Chapter 3) to be certain that the
viable types could not fail to include those in which the regular diploid chromosome
number had by chance been reassembled.*
That the production of an occasional viable spore from a sixteen-celled sporangium
* Since the above was written some positive evidence in favour of the primitive form of D. Borreri
having been diploid has come to hand by finding that the population of this species on the unglaciated
island of Madeira is exclusively diploid.
192
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
is not a negligible possibility is made clear by the behaviour of the two upper members
of the series (the tetraploids and pentaploids), which also provide such genetical evidence
as we possess. That they are hybrids between the lower numbered forms of D. Borreri
and the Male Fern is suggested not merely by their isolated occurrence, their chromo-
some number and their mixed morphology, but also by a curious circumstance affecting
the total spore output. This is always low, as may perhaps be seen in Fig. 201, in which
the relatively good spores of diploids and triploids contrast quite strongly with the high
proportion of shrunken spores found in tetraploids and pentaploids. So marked is this
character that inspection of the spore output of an unknown plant in which hybridity
of this kind is suspected can provide almost as reliable a guide to its chromosome
number as in other cases of hybrids in which apogamy is not involved. In these plants
there are, however, always a few very large good spores produced which, on sowing,
give a sparse crop of apogamous prothalli which reproduce the parental type exactly.
The low spore output is therefore presumably the reason why homogeneous local
populations are not formed.
The reason for the low spore output is at once to be seen on sectioning a sorus. Where-
as in diploid and triploid (with the exception of var. polydactyla Dadds) the eight-celled
type of sporangium is so conspicuous as to dominate the general field, in tetraploids and
pentaploids it is the sixteen-celled type which predominates to the extent that in one
tetraploid for which accurate counts of the two types were made, sixteen-celled out-
numbered eight-celled by 17:1. It is, therefore, a rather exceptional sporangium
which is able to carry through the normal apogamous development, and viable spores
are therefore correspondingly scarce.
This is a very curious type of genetical expression of a character. Apogamy, though
inherited, is not behaving at all like a simple Mendelian dominant, nor is it even a
partial dominant in the usual sense. Combination of an apogamous with a sexual
species might perhaps be expected to produce the intermediate type of sporangium
described as type 3 on p. 166 if dominance were imperfect, yet we find merely a
reduced percentage of eight-celled sporangia in which, however, the apogamous
qualities are fully developed.
This type of inheritance, it may be said in passing, provides strong confirmation of
the hybrid origin of the tetraploids and pentaploids under discussion (and perhaps also
of some triploids such as var. polydactyla Dadds), since it agrees exactly with what has
been found in a hybrid of known origin described and synthesized by Dopp. Dopp's
hybrid was between a diploid crested form of D. Borreri and normal D. Filix-mas, and it
must therefore have been tetraploid. The crested character proved to be recessive,
though the hybrid resembled D. Borreri in the shape of the pinnae and indusium. It
also resembled D. Borreri by reproducing apogamously, though the spore output was
low owing to a preponderance of sixteen-celled sporangia.
These facts are not only interesting in themselves but they provide an opportunity
for testing the possibility that occasional viable spores may be formed in the sixteen-
celled sporangia. An experiment of this kind was carried out on a pentaploid from
Ireland. This was lifted one autumn and transferred in a large pot from the mixed
population of the experimental garden to an isolated greenhouse about a mile away on
MFC ig2 ^^
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
the roof of the University in which no other sporing ferns were present. Next spring,
when new leaves had expanded, uncontaminated spores were collected and sown on to
soil after sterilization of soil and pot in an autoclave. During culture the pots were
kept covered with a glass lid and were watered only from below by standing them in a
zinc tray; a chance admixture of stray spores from outside seemed therefore excluded.
On germination, the usual sparse crop of apogamous prothalli was produced which
were removed to another pan as soon as they appeared. The residue was then closely
scrutinized. A few abortive apogamies were found, as described by de Bary, but, in
addition to these, five specimens were found with definite and fairly abundant arche-
gonia in the usual position. All these five looked more or less abnormal, recalhng indeed
the type of aberrant and depauperate prothalli which result from a sowing of spores
from triploid Osmunda. Attempts to induce the formation of sexually produced young
plants on them were unsuccessful and all died without offspring, although one remained
alive for two years making very slow growth before it succumbed. None showed the
slightest attempt at apogamous developments of the usual kind. With regard to their
nature it seems difficult to avoid the conclusion that they had originated from sixteen-
celled sporangia in which a minute proportion of viable but genetically unbalanced
spores had been formed. With a larger sowing the still smaller proportion of viable and
balanced types might have been detected.
The importance of this experiment is that it shows at least one way in which saltations
can occur in spite of apogamy, although to what extent, if any, these have actually
occurred in nature is unknown. It also demonstrates the importance of the nucleus in
the determination of this type of apogamy, since here we have what is tantamount to
segregation of sexual versus apogamous characters in the oflTspring of one individual, and
the peculiar genetical behaviour of apogamy can therefore not be explained by any
type of maternal cytoplasmic inheritance.
The genetical inference would appear to be this. Apogamy cannot be determined by
a simple Mendelian mechanism involving one mutant factor which is either dominant
or recessive. It seems rather to be the expression of a generalized unbalance of a quantita-
tive kind involving several (i.e. at least two and possibly many) different processes the
interaction of which is required for development of sporangia and prothalh, and some
or all of which are liable to fluctuate from environmental causes internal or external to
the plant. The exact fate of any individual sporangium is therefore to some extent
indeterminate, though the statistical frequency of the diflferent types of development may
be constant and characteristic for a given plant in a given environment.
Such a condition could perhaps be produced in a pure line by the simultaneous
occurrence of two or more genetical mutations of appropriate type; such an occurrence
is, however, in a high degree improbable. For this reason it need cause no surprise that
no case has yet been recorded of apogamy of this kind occurring as a local variant
within a pure species. Multiple unbalance of many genetical factors can, however,
readily occur at one step by the mating of gametes of different constitution, and it can
be no accident that interspecific hybridization has been suspected or proved for every
example about which sufficient information is available. That such hybrids often in-
volve the mating of gametes of different grades of polyploidy is perhaps only incidental,
194
APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES
since polyploidy as such is by no means confined to apogamous ferns; but that triploidy
may perhaps in some way predispose to the type of unbalance required is nevertheless
perhaps indicated by the remarkable preponderance of triploids among the species
analysed. No reason for this can at present be advanced, but the fact gives emphasis to
the general conclusion that, at least as regards the ferns, hybridization is (as was first
suggested by Ernst in another connexion) a cause and not merely the occasion for the
manifestation of apogamy, whatever the physiological mechanism may be.
One last conclusion is perhaps worthy of comment. The whole phenomenon of
apogamy of the type under discussion is a complicated departure from the normal which
is nevertheless repeated with almost monotonous identity of detail in every case which
has arisen. The somatic organization and normal development of sexual ferns is pre-
sumably such as to lend itself rather easily to this particular innovation, but whatever
may be the facts regarding this, a more striking example of parallel evolution would be
hard to find. This is of some importance, since we have already had occasion in
Chapter 6 to comment on the evidence for parallel evolution in the developmental
changes affecting soral structure and the form of the indusium. We seem forced to
conclude that wherever (for reasons known or unknown) certain types of change can
occur more easily than others, they will make their appearance repeatedly as long as
the structural conditions facilitating them prevail. This is perhaps the reason for the
taxonomists' perennial difficulties in tracing phylogeny and why, in the well-known
phrase, a phyletic 'tree' so often resembles less a trunk with branches than a bundle
of sticks.
SUMMARY
The following list summarizes the basic facts for each species mentioned in the chapter,
together with the country of origin of the actual material used:
Grade of
polyploidy
Diploid
Triploid
Tetraploid
Pentaploid
Incertae sedis
Species
Pteris cretica L.
Dryopteris Borreri Newm.
Chromosome
no.
58
82
Pteris cretica L. var. albolineata c. 90
Hook. (probably 87)
Dryopteris Borreri Newm. 123
D. remota (A.Br.) Hayek 123
D. atrata (Wall.) Ching 123
Cyrtomium falcatum (L.f.) Presl 123
C. Fortunei ].Sva. 123
C. caryotideum (Wall.) Presl 123
Asplenium monanthes L.* 108
{?)Pellaea atropurpurea (L.) Link 87
Pteris cretica L.
Country of origin
Italy
Britain and Switzerland
Ceylon and Hort.
Britain, Switzerland, Ger-
many, Norway
Britain, central Europe
Hort. (probably China)
Hort.
Hort. and China
Uganda
Madeira
California
Hort. and Uganda
Britain
Britain
Britain, Sweden
* Information added in proof on new material from Madeira.
c. 120
(probably 116)
Dryopteris Borreri (secondary 1 64
hybrids)
Dryopteris Borreri (secondary 205
hybrids)
Phegopteris polypodioides Fee 90
Derivation
Probably hybrid
Certainly hybrid
Probably triploid
Double the diploid
Hybrid with D.
Filix-mas
13-2
CHAPTER 12
INDUCED APOGAMY
In contrast to the species discussed in the last two chapters, in all of which apogamy was
a permanent feature of the life history, fixed and determined by a genetical mechanism
carried by the nucleus, it has long been known that normal sexual prothalH in a number
of species can be induced to become apogamous as an exceptional and temporary con-
dition if normal fertilization is prevented. One method of doing this, first used by Lang
in 1898, is of the simplest. The prothalli are grown on soil in covered pots standing in
water. The glass cover, which may conveniently be half a Petri dish placed over the top
of the pot, prevents the access of free water to the prothalli from above while permitting
ample moisture for physiological purposes to reach them from below. Under these con-
ditions no drops of free liquid are formed upon the prothalli, and the sex organs, al-
though fully developed, cannot open. Young plants cannot therefore be formed by
sexual means, though they will readily appear if the lid is removed and surface-water
supplied. The unfertilized prothalli will continue to grow and may reach an unusually
large size before irregularities of form become apparent, but when these begin apoga-
mously developed sporophytes may result by methods which, morphologically, differ in
many details from the sequence of events accompanying obligate apogamy, although
the end-result is somewhat similar, namely, a vegetative bud with leaves, roots and
ultimately a stem which may establish itself in the soil and grow on as an independent
individual. Two rather diflferent types of morphology were described by Lang, both of
which have been found again by subsequent authors. In many instances a cylindrical
process covered at first with archegonia may grow out from the region of the central
cushion ; the centre of this process may later become vascular, while leaves, roots and a
stem may develop from the tip. In other cases isolated organs from sporangia to leaves
and roots could proliferate from one or both surfaces of the prothallus without the inter-
vention of any special organ, or could grow out from the apex or margins. The most
surprising instance of such isolated organs were two cases (in horticultural strains of
Dryopteris and Scolopendrium respectively) in which sporangia, unaccompanied by other
sporophytic organs except sometimes protective scales, were borne directly on a
prothallus.
A list of the species used by Lang (1898) in the course of two and a half years is given
below, together with such of his morphological notes as refer to the physical characters
of the apogamous developments. As will be seen, a cylindrical process precedes the
formation of apogamous buds in the great majority of cases. It is perhaps to be regretted
that such a large number of the strains used were horticultural varieties and not the
typical wild species; this may perhaps have been of importance in determining the high
incidence of apogamy. Marked differences evidently existed in the ease of induction of
sporophytes in different varieties of the same species (cf Scolopendrium, Polystichum
angulare and Athyrium niponicum), suggesting that predisposing causes of a genetical kind
196
INDUCED APOGAMY
may have been, present in some cases, though of a kind quite unlike those operative in
the cases of direct apogamy previously discussed.
List of results obtained by Lang in 1898
Name Form of apogamy
Scolopendrium vulgare Sm. var. ramulosissi- Cylindrical process usually from the apical region of the prothallus.
mum VVoll. Tracheids in cylindrical process. Leaves, roots, sporangia and
ramenta on process. Vegetative buds from tip of cylindrical process
or in place of an archegonial projection.
var. marginale Similar to var. ramulosissimum but no sporangia, ramenta or leaves found.
JVephrodiu'm dilatatum Desv. {— Dryopteris Cylindrical process usually from the under surface just behind the apex
dilatata) var. cristatum gracile which formed a middle lobe. Tracheids in process and middle lobe.
Sporangia sometimes associated with ramenta on middle lobe and
process. No vegetative buds.
jV. Or^o/)fem Desv. (var. coro«a;;5 Barnes) Cylindrical process from apex of prothallus. Tracheids in process.
Ramenta on process. Vegetative buds rare.
Aspidium aculeatum Sw. ( = Polystichum acu- Tracheids in prothallus. Vegetative buds rare.
leatum) var. multifidum W'oU.
A. angulareWiVid. {= Polystichum angulare) Ramenta on prothallus. Vegetative buds frequent,
var. foliosum multifidum
No apogamy seen.
var. acutifolium multifidum
Athyrium niponicum Mett.
var. cristatum
A. Filix-femina Bemh. var. percristatum
var. cruciato-cris latum
var. coronatum Lowe
Polypodium vulgare L. var. grandiceps Fox.
Aspidium frondosum Lowe
Tracheids in prothalloid growths from archegonial projections.
Similar to normal form but in addition a few apogamously produced
vegetative buds.
Cylindrical process from apex or under surface.
Tracheids in process.
Continuation of process as a leaf. Vegetative buds.
Isolated leaflike growths. Vegetative buds numerous.
Vegetative buds on short cylindrical processes.
In addition to Lang's paper of 1898 the earlier Hterature includes observations by
Stange (1887) and Heim (1896), both containing observations on apogamy in species of
Doodia. Helm's paper is of particular importance as the first description of cyhndrical
processes in D. caudata. Later, in 1908, Yamanouchi described some early stages of
apogamy without cylindrical processes in 'Nephrodium molle\ but doubt was cast on his
results by Black in 1909, who attempted to repeat them but failed. Later still, in 1929,
Lang returned to the study of sporangia on prothalU through a repetition of the pheno-
menon in new material and gave some additional descriptive facts about it and about
apogamy as a whole in Scolopendrium. Lastly Duncan, working under Lang's direction,
repeated Helm's early work on Doodia caudata, adding some important new facts to it
(Duncan, 1941). These included, for the first time, some cytological observations on
apogamously produced plants, the two chromosome numbers found being given by
Duncan as c. 65 and c. 130 respectively. This confirms the reaUty of the fact of induced
apogamy, if confirmation were needed, and it also adds the last paper of importance to
the subject with which I am acquainted.
197
INDUCED APOGAMY
The aim of this chapter will not be to add anything to the morphological aspect of
the problems raised by induced apogamy, but merely to amplify the cytological know-
ledge relating to two historic examples already mentioned in the above lists, namely,
Doodia caudata (Cav.) R.Br, and Scolopendrium vulgare, as used by previous investigators.
The genus Doodia consists of a group of five closely related species of small tropical
ferns, spread from Ceylon to New Zealand and characterized by a soral structure closely
akin to Asplenium and Scolopendrium but borne on coriaceous evergreen leaves with the
outline depicted in Figs. 203 and 204. Doodia caudata (Cav.) R.Br, itself is native to New
Zealand and Australia. It was introduced into European botanic gardens in the middle of
last century and had been used for experimental purposes as early as 1887 by Stange. It
is not known whether all botanic garden stocks still in cultivation relate back to a
common source or whether the species may have been introduced more than once. Un-
fortunately, precise records concerning such matters have not as a rule been kept, and
therefore the only thing which can be said with certainty about the origin of the material
to be quoted below is that it came from Kew in 1938 and is identical with that used by
Duncan.
The morphological part of Duncan's work had involved the induction of apogamy by
Lang's method in normal prothalli of D. caudata, which was followed by the induction
of apospory on young, detached leaves laid on soil from both apogamously produced
and sexually produced plants. None of the new types of sporophyte was kept alive tor
long enough to reach fertility and therefore nothing was known about meiosis. Root-tip
counts of apogamous plants versus sexually produced plants were, however, successful
in demonstrating the gametophytic chromosome number in the apogamously produced
plants. This was assumed by Duncan to be the haploid number, but, as will be shown
below, this is probably not the case.
The material for the following observations has been supplied to me by Professor
Lang's former research assistant, Mr Ashby, without whom the present chapter would
not have been written. After Dr Duncan's departure from the Cryptogamic Laboratory
in Manchester, at the end of a year's visit, the old culture pans which had been prepared
for his work and which were still in being, were kept under observation by Mr Ashby,
and apogamously produced plants were extracted and carefully nursed as fast as they
appeared. A very beautiful specimen, detected and photographed by Mr Ashby, is
shown in Fig. 202 c, in which one old prothallus may be seen carrying two young
sporophytes. The small sporophyte on the right is apogamously produced, while the
larger one on the left is sexually produced from a belated fertilization after the apoga-
mous plant had already been initiated. Since the larger plant is also the younger, initia-
tion of the two in the reverse order being impossible, the difference of size is very clear
demonstration of the diminutive stature of the first-formed organs of an apogamous
plant. This makes cytological study somewhat difficult, since the roots of such plants are
of thread-like thinness. A fortunate specimen, however, gave the plate of chromosomes
shown in Fig. 202 e and/, and the very great difference in chromosome number between
this and a sexually produced sister can at once be seen by a glance at Fig. 202 b-d.
With regard to the chromosome numbers themselves, these have not yet been
determined with complete finality owing to my own departure from Manchester and
198
'. <
• V .
• • » » •
• • •• •
• ♦ #
■f^
/
Fig. 202. Induced apogamv in Doodia caudata (Cav.) R.Br. a. A prothallus bearing an apogamous
sporophyte (right) and a sexually produced sporophyte (left). Natural size. From a specimen
preserved in alcohol, b, c, d. Different focal levels through one root tip cell of a sexually produced
plant from a section, x .500. e, f. Two different focal levels through one root tip cell of aa
apogamously produced sister plant to show lower chromosome number, from a section, x 1500.
199
INDUCED APOGAMY
consequent interruption of the work. My estimates agree, however, closely with those
of Duncan which are certainly of the right order.
Fig. 203. Living leaves of normal Doodia caudata (Cav.) R.Br, of the strain used. Natural size.
By skilful cultivation, Mr Ashby succeeded where previous investigators had failed, in
keeping the apogamously produced plants alive for long enough to reach maturity and
Figs. 203 and 204 were obtained from fully mature fertile fronds of both types of plant
200
INDUCED APOGAMY
in 1947. The apogamously produced plants (Fig. 204) are still somewhat smaller than
their sexual sisters (Fig. 203), though this is only apparent on fairly close inspection. On
a casual glance the two types of plant are far more alike when mature than when young,
and without their labels they would be difficult to distinguish apart.
Fig. 204. Living leaves of an apogamously produced plant
oi^ Doodia caudata {Cav.) R.Bt. Natural size.
This close resemblance applies not only to the plants as a whole but also, somewhat
surprisingly, to their sporangia and spores. As may be seen from Fig. 205, the sporangia
of both are well filled with normal-looking spores, the only obvious difference being that
those of the apogamous plant are smaller in size. Somewhat disappointingly, however,
the spores of the apogamous plant have so far proved incapable of germination in spite
201
INDUCED APOGAMY
of their perfect morphology. They may all therefore be suspected of being genetically
unbalanced.
Meiosis has been studied as far as is possible in sections in both types of plant, and the
comparison, in view of their previous history and consequent pecuHar relationship, is
one of the principal points of interest about them. Fig. 206 a shows the first meiotic meta-
phase in a normal sexually produced plant, and Fig. 207 is of diplotene or very early
diakinesis in the only squash preparation which I had the opportunity of making.
Neither is suitable for detailed counting, although both show clearly the perfect regular-
ity of the division without either unpaired chromosomes or multivalent groups. Fig.
Fig. 205. Sporangia and spores oi Doodia caudata (Cav.) R.Br, x 100. a. An apogamously
produced plant, b. A sexually produced sister plant.
206 b, on the other hand, shows meiosis in one of the apogamously produced plants. The
much smaller size of the cells, no less than the irregularity of the metaphase plates, strikes
the eye at once. This is as expected. It might, however, also have been expected that if
this plant is indeed a haploid only unpaired chromosomes would be found, yet this is not
the case. Univalents are certainly never absent, but they represent only a small propor-
tion of the whole. Numerous pairs, visible on the equator even at the relatively low
magnification of the photograph, are present also, and it is even possible that there are
some multivalents. Pairing of this kind recalls somewhat the behaviour of the sixteen-
celled sporangia of Pteris cretica, and the most probable interpretation would appear to
be, as in that species, that the chromosome number of the apogamously produced plants
of Doodia, though gametophytic, is not haploid. We seem to be deahng with diploid
derivatives from a tetraploid stock which was itself allopolyploid. Or in other words,
with a strain of Doodia which, beneath its regular pairing achieved by a chromosome
doubling which the conditions of the experiment have reversed, contains two different sets
of chromosomes between which considerable though not complete homology exists.
Two gametic sets of this type are likely to be derived from distinct though related
species, and we reach the somewhat unexpected conclusion that the D. caudata used by
us and in cultivation at Kew is another case of a concealed species hybrid.
202
INDUCED APOGAMY
It would be a matter of considerable interest to be able to examine cytologically some
freshly collected wild examples of the species, since the history of cultivated plants is such
that it would be unwise to exclude the possibility that hybridization and subsequent
chromosome doubling may have occurred in a botanic garden and not be a specific
attribute of Z). caudata in the wild state. Until this can be done it is scarcely profitable to
study these plants in greater detail. These results are nevertheless of interest in showing,
first, how an unsuspected hybrid can be detected by a technical method different from
those previously employed. Secondly, it is perhaps appropriate to notice that had any of
F
<
\-
r
r
\^
..^ ^M
Fig. 206. Meiosis in Doodia caudata (Cav.) R.Br, from sections, x 1000. a. Sexually produced plant
with regular pairing, b. Apogamously produced plant with many lagging univalents but also
some pairs.
the spores borne by the apogamously produced plant been viable, which under certain
conditions might conceivably occur, this species might have given one of the rare
examples of an experimental series advancing in the direction of diminudon of chromo-
somes. The fact that this possibility has not been realized, though expectation has been
brought near enough for it to seem conceivable, brings out very clearly the relative irre-
versibiHty of most types of polyploidy hitherto encountered, and this irreversibility,
whatever its cause, is undoubtedly a fact of evolutionary importance.
Before leaving Doodia it may be worth mentioning in passing that Dr Duncan's results
on apospory were also repeated by Mr Ashby, though less attention was given to these
and the plants have not been cultivated to maturity. The consequence of inducing
apospory in an apogamously produced plant is, of course, merely to return again to a
normal prothallus. The consequence of inducing apospory in a normal young plant is
the production of a polyploid prothallus. One such prothallus after self-fertilizadon gave
a sporophyte, in the roots of which over 200 chromosomes were counted. Since twice
203
INDUCED APOGAMY
140 should be 280, this number may be thought of as octoploid if the normal plant is a
tetraploid. Unfortunately the only specimen died as a result of attempted transfer
from Manchester to Leeds and nothing more can be said about it. It would be a matter
of some interest to repeat the induction of such a plant in order to determine whether or
not it would be capable of becoming fertile. It would be expected to show marked signs
of abnormality and of sterihty if the grade of polyploidy imputed to it is genuine.
With Scolopendrium as with Doodia I am fortunate in having had access, through the
vigilance and private enterprise of Mr Ashby, to residual material remaining in culture
from an earlier investigation, the material in this case being that relating to Lang's
second paper (1929) on 'apogamy and the production of sporangia on prothalH in
Fig. 207. Meiosis (early diakinesis) in normal Doodia caudata (Cav.) R.Br,
to show regular pairing, permanent acetocarmine. x 1000.
Scolopendrium'. The strain of Scolopendrium used had originated as a stray spore of
unknown origin which had germinated in the moss house in Manchester University
Experimental Grounds and given rise to a plant with characteristic, heritable, leaf ab-
normalities of the type so often seen in horticultural strains. The leaf morphology, which
can to some extent be seen in Fig. 208, could have been described as 'ramo-furcate', and
in addition to the forking of the rachis and profuse branching of the leaf apex, there was
a tendency to produce sori on the upper as well as on the lower surface of the leaves. The
prothalli obtained from spores of this plant also showed a somewhat comparable pecu-
liarity in the frequent production of archegonia on both surfaces, but they were other-
wise quite normal and readily gave rise, when suitably watered, to crops of young plants
with a morphology exactly similar to that of the original specimen. The morphological
characteristics of both generations therefore appeared to be genetically determined.
When water was withheld, in the manner already described, the unfertilized prothalH
became very large and more abnormal in appearance. One sign of incipient apogamy
204
INDUCED APOGAMY
was the spasmodic appearance here and there in a culture pan of small groups of pale-
coloured sporangia borne on the upper surface of some though never on all the prothalli,
and at a later stage these were followed by the apogamous development of other organs
from different parts of the prothalli.
The residual material remaining from Lang's work consisted of a number of adult
sexually produced sporophytes of the strain and one old culture pan of prothalli, which
had, however, apparently ceased to produce sporangia. In order to rejuvenate the pro-
thallial material fresh spores of the strain obtained from one of the descendants of the
original plant were sown in 1943 and kept away from free water for over a year. Late in
Fig. 208. Sexual and apogamous Scolopendrium vulgare Sm. a. Sister plants of the same age, of the horti-
cultural strain used, the larger plant produced sexually, the smaller apogamously. About one-third
natural size. b. Root-tip cell of a sexually produced plant from a section, x 1000. c. Root-tip
cell of an apogamously produced plant from a section, x 1000.
1944 the first signs of apogamy were encountered, including both apogamous buds and
sporangia on prothalli, and their production continued sparingly during 1945 when
observation ceased. During this time one group of sporangia was successfully sectioned
by Mr Ashby and meiosis in it encountered, while other specimens of apogamous out-
growths were lifted from the pan and grown on. One complete plant obtained in this
way survived for long enough to give the photograph of Fig. 208, which was taken in the
autumn of 1 945. This plant was sparingly fertile for the first time in that year, producing
two diminutive sori, both of which were used for cytological purposes, after which the
plant unfortunately died.
The relative difference of size between a sexually produced and an apogamously pro-
duced plant of equal age of this strain o^ Scolopendrium is clearly shown by Fig. 208 a. In
the younger stages the difference is still more marked, the organs of apogamously pro-
duced plants being so small and delicate that they are difficult to handle. As expected,
the cytological basis for this difference of size lies in difference of chromosome number.
The sexually produced plants, in spite of their ramo-furcate and other qualities, have the
normal chromosome number of the wild species and show 72 chromosomes in their roots
205
INDUCED APOGAMY
(Fig. 208^) and 36 pairs at meiosis (Figs. 209^, 210). In the little roots of apogamous
plants there are, however, only 36 chromosomes (Fig. 208c).
The behaviour of these 36 chromosomes at meiosis is one of the principal points of
interest in this material, and though the actual number of mother cells available has
been very small indeed, being restricted to the two sori and one group of sporangia on a
prothallus above mentioned, enough has been seen for a few statements to be made. From
the two mother cells visible in Figs. 209 b, and 210b and c, at the stages of metaphase and
interkinesis, it is clear that meiosis is of the most irregular type possible. Chromosome
pairing (Figs. 209^, 210 b) is either completely absent or so shght that it cannot be de-
tected without a larger number of cells for inspection. Chromosome distribution at
^P I
•">. -H
Wl
n
Fig. 209. Meiosis in sexual and apogamous Scolopendrium vulgare Sm., permanent acetocarmine. x 1000.
a. Sexually produced plant showing 36 pairs, b. Apogamously produced plant showing two
mother cells and tapetal nuclei. For explanation see text and Fig. 210^ and c.
anaphase is apparently at random, and is most unequal, so that very dissimilar-sized
daughter nuclei are present in the succeeding resting stage (Figs. 209^, 210c). I have not
seen the second meiotic division, but the result of meiosis is total abortion of the spores.
Such behaviour might have been expected and has indeed already been looked for un-
successfully in this and the two preceding chapters. The conclusion seems undoubtedly
to be that here at last we have a genuine haploid fern, comparable to the various well-
known cases of haploid sporophytes obtained in Flowering Plants but which, in the
Pteridophyta, had appeared to be curiously elusive. The failure of pairing among the
various chromosomes is almost certainly due to lack of homology between them, which
is to be expected in a genuine monoploid set. That « = 36 has, indeed, been shown to be
a fundamental monoploid condition in several related genera {Asplenium, Ceterach, Scolo-
pendrium) makes this conclusion the more probable, and therefore for once we may discard
all suspicion of harbouring a concealed hybrid or polyploid and accept the demonstra-
tion that apogamy provides that Scolopendrium vulgare is a simple diploid species without
any ambiguity.
It would, however, be interesting to know still more about it. A repetition of these
observations on normal forms of the wild species might not be impossible and would be
of great value if it resulted in more abundant meiotic material. This would have more
206
INDUCED APOGAMY
than a merely confirmatory usefulness. It is a well-known fact that in many genuine
haploids a limited power of pairing may be conferred on essentially non-homologous
chromosomes by small structural lesions such as reduplications or translocations of re-
duplicated segments. These structural changes within chromosomes may, or may not,
alter the genetical content of the nucleus as a whole, and they may or may not be
detectable in the external morphology of the plants. The evolutionary importance of
structural changes within the monoploid set is, however, far greater than their imme-
diate visible effects might suggest. They introduce changes of behaviour of a wholly
X
;$♦»' ^tr ^
V
a
Scolopendrium n - 36
Fig. 2IO. Explanatory diagram to Fig. 209 a and b. x 1500.
different type from those so far studied in the Pteridophyta, and any practicable means
for bringing them effectively under observation is much to be desired.
So much is this the case that it may be suggested that one of the most fruitful lines of
inquiry at present outstanding in the cytology of the British fern flora is the extension of
induced apogamy to other wild species besides Scolopendrium. That this might not be im-
possible is suggested by the relatively large number of species represented on Lang's
original list, and if it could be done on any scale, more insight might in a short time be
gained about the cytogenetic composition of our native species than would be expected
to be reached in any other way. It is therefore much to be hoped that this work will be
extended.
SUMMARY
The induction of apogamy in two well-known horticultural strains of Doodia caudata
(Cav.) R.Br, (the strain used by Duncan, 1941) and Scolopendrium vulgare (the strain used
by Lang, 1929) has been repeated by methods previously described by earher workers
with the object of including cytological observations on them. In both cases the apo-
gamously produced plants were raised to reproductive maturity and meiosis seen. In
207
INDUCED APOGAMY
Doodia caudata the pairing among approximately 70 chromosomes present is so high as to
suggest that though gametophytic this plant is not a haploid but a diploid derivative of
an allotetraploid strain. In Scobpendrium vulgare, on the other hand, pairing in its 36
chromosomes is so low as to be quite absent in the very few cells which have been seen.
In this case, therefore, alone among all the examples of apogamy so far examined, we
seem to have a genuine haploid sporophyte. S. vulgare itself may therefore be accepted
as a genuine diploid species without any ambiguity.
208
CHAPTER 13
THE GENUS EQUISETUM
Turning now away from the ferns to consider some representatives of the other great
groups included with them in the Pteridophyta we may take as our first example of a
' microphyllous ' (small-leaved) group, the genus Equisetum.
The very curious appearance of the Horsetails has already to some extent been illu-
strated in Chapter 2 and other examples will ht found on p. 211 and on p. 226. So
striking are they that most European languages are rich in popular names for them, and
they are familiar playthings of most country-bred children, at least in the west of the
Continent. Their relationship to the ferns is by no means obvious to the layman, and it
rests, indeed, primarily on community of hfe history and on the structure of the sexual
generation, details which can scarcely be observed outside a laboratory. An affinity with
the Lycopods or 'clubmosses' is perhaps easier to detect, both groups possessing small
leaves in contrast to the 'megaphyllous' (large-leaved) ferns, but closer inspection re-
veals so many differences in anatomy, morphology, form and position of the reproduc-
tive structures of both generations that the relatively isolated position of the Horsetails
has long been recognized by assigning them to an independent group, the Equisetales, of
equal rank to the Fihcales (ferns) and Lycopodiales (clubmosses) and of an antiquity at
least commensurate with these.
The Equisetales have existed almost from the earliest times at which fossil plants have
been preserved. From beginnings traceable with difficulty in the rocks of the Devonian
they achieved in the Coal Measure period a burst of evolutionary development which
they never repeated. The Calamites of the Coal Measure forests were large trees present
in great numbers, both of individuals and of species, while the variety of cone structures
actually found must denote the existence of a far greater number of generic types which
are lost and which greatly exceed anything that the one living genus might lead one to
expect. That the range of early genera may have included Equisetum itself side by side
with its arboreal relatives is suggested by the finding of a small fertile specimen of
Carboniferous age described under the name o{ Equis elites Hemingwayi by Kidston (1892),
in which the cone structure is identical with that of the living genus except for the larger
size of the sporangia and scales. Unfortunately, the specimen only shows the external
form of the plant and not the anatomical structure, without which the generic identity
with living species is uncertain; nevertheless, the evidence is clear enough to suggest
that Equisetum itself may extend in unbroken sequence from the Coal Measures to the
present day, and if indeed it did so, we should have to regard it not merely as the sole
living representative of a large and ancient group but as itself the oldest living genus of
vascular plants known to science. For this reason alone a comparison of its cytological
constitution with that of the recent ferns is likely to offer points of unusual interest.
In contrast to the age of the genus and the wealth of extinct species traceable at many
geological levels above the Carboniferous period the existing species, some two dozen in
Mpc 209 ^*
THE GENUS EQUISETUM
number, are virtually devoid of direct fossil history. Nevertheless, signs of age are not
lacking from the evidence of geographical distribution. The total distribution of the
living genus is almost worldwide, though Australia and New Zealand contain none and
there are relatively few species in the southern hemisphere and in the tropics. With this
proviso it is striking that at least half of the living species have a distribution which
straddles the entire range of the genus, and all the European species are at least circum-
polar. Only in America can there be found any considerable number of species which
do not extend beyond that continent. These facts are perhaps most clearly revealed by a
citation of the general distribution of the European species, and since only two of these
are non-British and all are quoted in Hegi's Flora, the information easily obtainable from
this source is reproduced in the following list :
Species General distribution
E. sylvaticum L. North and central Europe, north Spain, Balkans, north Asia, North America.
E. pratense Ehrh. British Isles, north- and east-central Europe, Caucasus, Siberia, North America.
E. maximum Lam. Europe (except Scandinavia and a large part of Russia), west Asia, western North Africa,
North Atlantic Islands, western North America.
E. arvense L. Europe, north Asia, North Africa, Canaries, South Africa, North America.
E. palustre L. Europe (except for parts of the Mediterranean region), Caucasus, temperate Asia, northern
North America.
E. limosum Willd. Europe (except for parts of the Mediterranean region), north Asia, North America.
E. ramosissimum Desf. South and central Europe, temperate Asia, India, China, most of Africa (including Mada-
gascar), America both north and south.
E. hiemale L. Europe (except parts of Mediterranean region), north Asia as far as Japan, North America.
E. variegatum Schl. Europe (except for the true Mediterranean region and certain parts of the Danube countries,
Russia and Denmark), Siberia, North America.
E. scirpoides Michx. North Europe (Iceland, Spitzbergen, Scandinavia), Siberia and northern and arctic
America.
Outstanding in this list are E. arvense and E. ramosissimum, which are not only circum-
polar but present in the southern hemisphere also. Other interesting distributions are
those of £■. maximum and E. scirpoides. The former, though circumpolar in total extent, is
discontinuously so to an extreme degree, being present on the west sides of Eurasia,
Africa and America, but not on the east of these continents. This strongly suggests
regional extinction on a large scale, an event which can scarcely be entirely recent.
Direct signs of local extinction are given under somewhat different circumstances by
E. scirpoides (Fig. 211). This is not a British plant, having on the whole a more northerly
distribution, being met with right into the Arctic circle in Europe, Siberia and America.
It has, or perhaps had, in addition, one outlying station in central Europe, namely, at
Heihgenblut near the Pasterze Glacier in Austria. It was well known in this locality a
hundred years ago, though it has not been seen recently and may have died out. Since
this species is inconspicuous and of no economic or horticultural importance, it is im-
possible to imagine that either its introduction or its extermination, if it is in fact exter-
minated, in a station many hundreds of miles away from its main areas of occupation
can owe anything to human interference, and it therefore seems necessary to suppose
that both events are indicative of important changes of climate. E. scirpoides in central
Europe seems, in fact, to be a relict from glacial times, and if this is correct, one must
210
THE GENUS EQUISETUM
believe that the species once had an even wider range than it shows at present. All these
facts indicate a high degree of antiquity for the living species of Equisetum, quite apart
from the fossil record of the genus.
All the European species have been available to me for study of both mitosis and
meiosis. In addition, a very large Horsetail with sparsely branched shoots as thick as
one's finger and reaching 15 ft. in height was sent to me from Glasnevin Botanic Garden
Fig. 211. Silhouette of a live plant o( Equisetum scirpoides Michx. grown in cultivation
but originally from Norway. Natural size.
in Dublin, and subsequently identified by Mr Alston of the British Museum as the North
American species E. robustum A.Br. It proved easy to cultivate and in time bore cones.
Lastly, sterile and fertile material of three other forms, not strictly speaking species
although frequently treated as such in Floras, have been examined. These forms are
E. trachyodon A.Br., E. Moorei Newman and E. litorale Kuhlw., and they will be dealt with
separately at the end of the chapter.
21 1
14-2
THE GENUS EQUISETUM
A prerequisite to the effective study of any of these species is a fairly detailed know-
ledge of its coning habits. In a few cases cones could be obtained in culture,* but in a
greater number very close observation of wild populations was needed as the only means
of obtaining suitable fixations at the right time of year. Since the knowledge so gained
may perhaps be of interest to field collectors, being of a kind not easily obtained from
books, it may perhaps be of interest to append a few notes about it before considering
the cytological results. The seasons at which successful fixations were actually taken may
be listed f as follows :
Species Locality
Subgenus Eu-equisetum
Month of meiosis
E. arvense L.
Manchester district
September
E. maximum Lam.
>> j>
99
E. sylvalicum L.
3J )?
99
E. prateme Ehrh.
W'estonbirt School, Gloucestershire
J>
E. palustre L.
Manchester district
May
E. limosum L.
)> >)
?»
E. litorale Kuhlw.
Eastern Ireland
Subgenus Hippochaete
June
E. ramosissimum Desf.
Italy (Apennines)
July
E. hiemale L.
Newcastle district
September
E. robustum A.Br.
Cold greenhouse
J J
E. variegalum Schleich
Southport sand-dunes
May
Royal Dublin Canal
June
E. trachyodon A.Br.
Eastern Ireland
99
E. scirpoides Michx.
Pot culture
September
E. Aloorei Newman
Eastern Ireland
August
At first sight these dates may appear somewhat arbitrary, but it is not difficult to
correlate them with some fairly simple general habits of the various species. Thus all
those species with deciduous aerial shoots on which the cones are also borne will show
young sporangia with maturing mother cells on the newly emerging green stems in the
spring, that is, in about the month of May in Great Britain. The principal representa-
tives of this type are E. palustre and E. limosum (cf. Fig. 5, p. 18), in both of which the
young cones can indeed be found in the resting buds of the previous autumn but only
in a very immature condition. The non-deciduous species, e.g. E. hiemale, E. variegatum,
E. Moorei, E. trachyodon, and probably E. scirpoides — though I have not seen this non-
British species in its native haunts — mature their cones later in the summer or during a
longer season. Thus meiosis can be obtained under normal climatic conditions in all of
these species at the end of August, though in some, notably E. variegatum and perhaps
E. hiemale, a continuous succession of new shoots, some of them bearing cones, may be
* Most of the species listed may be grown easily in ordinary garden soil without any special treatment
except reasonable freedom from desiccation. My own plants were grown in the ground in an unhealed
partially shady fern-house, the only precaution taken being to enclose them in brick compartments
lined with cement to prevent the rhizomes from escaping into other parts of the house, since they
become very troublesome weeds if allowed to become rampant. Under these conditions vegetative growth
was luxuriant but cones were rare, probably through lack of light. The small species, e.g. E. scirpoides
and E. variegatum, wall, however, cone quite readily in pot culture, and E. limosum coned annually when
grown in a large pot submerged in an artificial pond. It is probable that E. palustre might do the same
though this species was not actually grown.
t The classification adopted here is primarily that of Milde (1867).
212
THE GENUS EQUISETUM
found from June to September. E. ramosissimum, which I have only seen wild in north
Italy, seems to be somewhat exceptional in that there appears to be a succession of cones
Fig. 212. Equisetum maximum Lam. in late September, showing two cone buds and the base of a sterile
aerial shoot all borne on the same node at the base of a sterile shoot of the previous year, from
a living specimen. Natural size.
during a long season, not on new shoots springing up from the rhizome but on the tips
of branches of different order on the aerial stems. Thus my own material was fixed in
the Apennines in July in manifestly late cones borne on the tips of lateral branches, the
213
THE GENUS EQUISETUM
main branches from which the laterals were emerging having presumably been fertile
earlier in the year.
Fundamentally different habits are met with in those species which bear their cones
either on separate short, colourless shoots or on special colourless apical portions of
green shoots. These species are E. arvense, E. maximum, E. pratense of the first type and
E. sylvaticum of the second. All these four species shed their spores very early in the
spring (March or April), in the first three cases before any green vegetative shoots are
visible, and maturation of the spores in each is completed in the previous autumn.
Since the cones of these species are perhaps less familiar in autumn than they are
in spring, a photograph, natural size, of E. maximum, dug up in late September, is re-
produced in Fig. 212. At this time structurally perfect green spores are already present
in the fat winter buds which enclose and protect the dormant cone under many sheaths
of russet brown scales. The buds, two of which are seen in the figure, project an inch
or more above the ground, and are as a rule to be found near the base of a standing
green shoot of the current year. The base of this is seen to the left of the cone buds,
and if both it and the buds are traced down to their origin it can be seen that both
arise as lateral branches from the base of a still older shoot of the previous year. The
habit of spring shedding of spores seems therefore to represent a delay in the liberation
of spores which morphologically belong to the year in which meiosis occurs rather than
to precocious development.
Meiosis takes place in these cone buds in early September, when they are already
above ground but not at their full size. The cone is easily exposed by sphtting the bud
or by peeling off the sheathing leaves. It is soft, white and juicy when meiosis is occur-
ring, becoming yellow with greenish sporangia when the spores are complete. In the
young stages the cones seem to be very attractive to small animals which I take to be
mice, since they are often found partly or completely eaten away.
Although the cone buds in all four species {E. arvense, maximum, pratense and sylvaticum)
are actually above ground at the time of meiosis and may be found readily enough by
careful examination of the soil surface near the base of a standing shoot, it is in practice
unprofitable to do this unless a coning site has first been marked down with some accur-
acy in a previous spring. In every species very large areas of ground may be occupied
by vigorous but sterile populations. Good coning sites seem to be topographically deter-
mined as places with rather better drainage and fuller illumination than those which
suffice for vegetative growth. This is perhaps the reason why E. arvense is so often found
to cone abundantly on railway banks. Once a coning site has been detected, fertile plants
will recur there year after year with unfailing regularity, unless the ecological conditions
alter. The problem of finding fertile material of E. arvense, maximum, pratense and
j^/ya^icM/n is then not difficult.*
Within the cone itself the order of maturation of the various parts is the same in all
species. The first sporangia to mature are those in the widest part of the cone, and de-
* The sites actually used for this work were : for E. arvense, a dump of disused builders' sand in a meadow
near Stockport; for E. maximum, the raised bank of a canal near Stockport; for E. sylvaticum, a clearing in
a steeply sloping wood near Stockport ; and for E. pratense, a well-drained east-facing rock garden in the
grounds of Westonbirt Girls' School, where this rare species has become established and cones annually.
214
THE GENUS EQUISETUM
velopment proceeds on the whole basipetally, although some young stages also remain
at the extreme apex. This is a very different sequence of events from that followed by the
leaves of a vegetative bud, and is perhaps an additional reason for thinking that the
'sporangiophores' may not be foliar in nature. Since even in the tiny cones of £. scir-
poides a considerable number of sporangia are contained, it is usual to find every stage
of meiosis in one cone, and quite a range of stages in one sporangium. The reason for
this is that the mother cells do not form a uniform tissue but are separated into compact
pockets of a few mother cells surrounded by plasmodial tapetum. Every cell of a pocket
will be at the same stage, but adjacent pockets may be quite widely separated both in
space and in stage of development. This may perhaps be seen in Figs. 5^ and 217^.
All species of Horsetail respond to normal methods of modern fixation excellently and
give very beautiful preparations, as the photographs in the pages which follow will per-
haps testify. This is a very fortunate circumstance, because even with this advantage we
step, with Equisetum, into a totally different order of difficulty from anything which we
have so far met with in the ferns. This difficulty is reflected in the hopelessly discordant
results obtainable from the literature, which may be quoted more as a warning than as
an example. Thus Tischler (19350) lists 2n = 24 for E. limosum (Steinecke, 1932);
2n = c. 30 for E. arvense (Lenoir, 1926) ] 2n = c. 140 for E. palustre (Lenoir, 1932), while
de Beer (191 3) gave n = c. 1 15 for E. arvense, and very recently Hagerup has reported
(see Love and Love, 1948) 2/2 = 230 for the majority of European species.
In my experience all these records are wrong though in differing degrees, the last two
being slightly too high but all the others being very much too low. The main cytological
difficulties in estimating chromosome number here are three — the number itself is high,
the chromosomes are very varied in size and shape, but, thirdly, their shapes are so
peculiar that at first sight of meiosis they may be very misleading and may even suggest
the presence of multivalent pairing, as a comparison of Figs. 2 13 ^ or 222 c with the photo-
graphs of polyploid Biscutella on p. 8 will perhaps demonstrate. The meaning of these
curious appearances is only imperfectly understood, though it seems to lie in a peculiar
weakness of the spiral structure in which the gyres tend to fall apart rather easily under
the natural stresses of the forces at work in the cell, although as may be seen from Fig.
228, p. 230, the spiral can be quite normal in unpaired chromosomes. This type of be-
haviour I have seen occasionally in otherwise normal plants of Osmunda (Manton, 1945)
under special metabolic conditions, and it is therefore unlikely to denote any very funda-
mental difference between Equisetum and other plants. The closely similar behaviour of
certain Lycopods may, however, perhaps denote a measure of phyletic affinity with that
group.
Before discussing the numerical evidence in detail it may be well to glance through an
array of preparations of different stages in different species. These are contained in the
photographs of Figs. 213-223. Diakinesis, metaphase i and anaphase 2 in four species
of the subgenus Hippochaete are contained in Figs. 213-216. Precisely comparable pre-
parations of two species of the subgenus Eu-equisetum are contained in Fig. 222<^ and c.
A marked difference of chromosome size may be noted in comparing these two groups of
figures. This difference seems to be characteristic of the two subgenera, all the forms ex-
amined of Eu-equisetum, namely, E. limosum, palustre, arvense, pratense, maximum, sylvaticum
215
^ ■*»•' .^
\
^^y
r\ "^^
3»v *.»
-*7
^
t - -^ •^ ^ ^*j
'A
% I.. » ^ • *
4^^^
Fig. 213. Permanent acetocarmine preparations of two large-chromosomed species of horsetail, x 1000.
For explanatory diagrams see Fig. 214. In both «= 108. a. Biakinesis in Equisetum robustum A.Br.
from America, b. First meiotic metaphase in E. variegatum Schleich. from Britain (Southport).
*\>
'^•^.'^ >
f.rcbusfuw n = cu. /OS
4. *^
5. variegatum n ^ 108
b
Fig. 214. a. Explanatory diagram to Fig. 213a. X 1500.
b. Explanatory diagram to Fig. 213^. x 1000.
THE GENUS EQUISETUM
and litorale, having smaller chromosomes than members of the other subgenus, which,
in this study, have been represented by E. variegatum, hiemale, scirpoides, ramosissi-
w
% «^% m^ ?. '•^
« «
■IT / *
Fig. 215. Permanent acetocarmine preparations of two additional large-chromosomed species of
horsetail, x 1000. In both 7; = almost certainly 108. a. Equisetum hiemaleh.hom Northumberland,
the first meiotic metaphase. For explanatory diagram see Fig. 216. b. E. scirpoides Michx., the
second meiotic anaphase to show shapes of the chromosomes at this stage.
mum, robustum, trachyodon and Moorei. Though the subgenera are primarily distinguished
by the structure of their stomata, the agreement between this character and relative
chromosome size suggests that the subdivision is a natural one and of long standing.
218
-1*
?«»*
£. hiemale n -- /08
Fig. 216. Explanatory diagram to Fig. 215a. x 1500.
F
■
1
■
f
»
•^** 4
w
**.
rl
%
■H
K
•*
k
1
f«JI--lllt,
Fig 217 Comparison of chromosome size at diakinesis in a large and a small chromosomed species of
horsetail, from sections, x 1000. a. E. arvense L. (see further Fig. 218). b. Equisetum robustum
A.Br. (cf. Fig. 213a).
-i*»-
Jr
_ < w
"»"
%
^ ■»
t
<
> ^'^ S >
<
/ _
».
♦ *
*" «
-i
«
y ■
. *'
¥
-► ♦ % *
*
* #
»
»
J
« ♦
«
• •
*» \
ft '^
* , <H
* >
• ' *
•
Fig. 218. Meiosis in Equisetum arvense L. in two different techniques photographed at the same magnifica-
tion. X 1000. a. A Feulgen squash in balsam. For explanatory diagram see Fig. 219. b. Afresh
mount in acetocai'mine. For description see text.
THE GENUS EQUISETUM
A further illustration of both absolute and relative sizes of the chromosomes at a com-
parable stage in one species of each of the two subgenera is given, from sectioned
material, in Fig. 217. In the upper photograph, showing diakinesis in E. arvense, the
chromosomes are distinctly smaller than in the lower photograph of E. robustum. This
demonstration has been included partly for comparison with sectioned preparations
illustrated in other chapters but also because Equisetum represents an extreme example
of the extent to which chromosome size can be affected by different technical treatments
1^
V t
i^. arvense n = ca. 108
Fig. 219. Explanatory diagram to Fig. 2i8fl. x 1500.
without the production of any other visible distortion. The fact that acetocarmine
almost doubles the apparent dimensions is one of the many reasons for the great useful
ness of this reagent, but a very misleading impression could be produced by mixing one's
techniques if it were wished to do so. Thus Fig. 218 shows on the same page two cells
of the same species, E. arvense, already illustrated in Fig. 217a, photographed at the same
magnification ( x 1 000) . The upper figure is from a Feulgen squash mounted in balsam.
The lower figure is from a fresh acetocarmine squash photographed in that liquid. The
exceptional clarity of the latter specimen was due partly to the fact that it had become
spread in close contact with the glass of the cover-slip, and the optical disadvantages of
observation in a liquid of low refractive index were therefore absent. The cell was
also exceptional in the extreme degree of enlargement which the chromosomes had
221
THE GENUS EQUISETUM
undergone. Some of this enlargement is reversible and would have disappeared had the
specimen been successfully transferred to balsam, but it unfortunately became detached
and was lost in the attempt, and a second photograph more comparable to the other
could not therefore be taken.
The numerical evidence from these and other preparations is as follows. All the
species of Equisetum examined have the same chromosome number to a very close
approximation. This falls certainly between the limits of io6 and 112, the haploid
chromosome number being quoted. It is probable, indeed almost certain, that the
chromosome number of all of them is actually 108.
\
^ %
■
•
•It
j
m *
%
^ s *
* i^^^^^^SL
Tm
0
' 1
•1
•
#
Fig. 220. Feulgen squash in acetic acid of meiosis in Equisetum maximum Lam. x 1000.
For further information see Fig. 221.
There are, however, two difficulties to which attention should be drawn before this
number is accepted. In all the large-chromosomed species, that is, in the subgenus
Hippochaete, there is a tendency for counts to exceed this number by one or two. For
these therefore the number 1 08 is a minimum value. In the small-chromosomed species
of the subgenus Eu-eguisetum, on the other hand, the difficulty is the other way. For these
the number 108 is a maximum, and one is often tempted to wonder whether the correct
value is not perhaps one or two less. The reason for this is perhaps psychological, in that
large chromosomes are sometimes more easily misinterpreted than small ones in the
direction of excessive subdivision, since very odd-shaped groups can deceptively re-
semble two pairs in accidental contact. An example of such a case is marked by an
arrow in Fig. 214a, and a very large figure-of-eight pair on the left of the same nucleus
222
THE GENUS EQUISETUM
could be similarly interpreted ; this cell could therefore possess a maximum number of
I lo. On the other hand, with small chromosomes of rather unequal sizes it is perhaps
more likely that a tiny pair lying near another might be falsely regarded as part of it.
Examples of doubt of this kind are shown by the arrows in Fig. 219, which refers to the
Feulgen squash oi E. arvense in which the maximum number is 108 though it could be
107 or 106. That the last alternative is incorrect is, however, proved by Fig. 218^. In
this there are also two doubtful places, but the range of possible numbers is 1 07-1 10.
The correct interpretation of this cell is probably that the upper doubtful place is
actually one pair with the halves pushed somewhat asunder, while the lower doubtful
' ♦ ' ^ f , /
/
Fig. 221. Drawing of the cell of Fig. 220 made whilst it was still fresh, x 1000. In addition to the 107
bodies shown there were four small granules which could have been the arms of a dismembered pair.
place is composed of two rather more compact and smaller pairs held deceptively near
one another by their attachment to the surface of the nucleolus. It must be recognized,
however, that a genuine difference of one or two chromosomes between Eu-equisetum and
Hippochaete would be very difficult to demonstrate and is therefore almost equally diffi-
cult to disprove. Should it exist, all these estimates will need slight revision, but if it
does not the only number which will fit the facts for every species is n = 108.
The general uniformity of these cytological results coupled with the signs of antiquity
to be found among the living species might, at first sight, lead one to suspect that
evolutionary activity within the genus Equisetum is perhaps becoming exhausted. Direct
evidence that some at least of the raw materials of evolution are nevertheless present is,
however, provided by the surprising number of interspecific hybrids which have been
discovered in Ireland. That this small country is specially favoured in this respect is
probable from the known rarity of Horsetail prothalli in most other countries. No
actual records of gametophytes exist for Ireland, but it may safely be assumed that its
mild, moist, oceanic climate is likely to favour their production.
223
THE GENUS EQUISETUM
The three cases of hybrid Equisetum available to me were all provided either by, or
with the help of, Dr Praeger of Dublin. Two of them, E. litorale and E. trachyodon, were
known to be hybrids from the literature, the third, E. Moorei, was expected to be a pure
4F#
f^r^c^,
^ i^^^JIt- €
* it %t I
;■' ^^ ^i
I ^s^ -^
#1
^. "^
^r
■•* "^
^**^
,^
»J5ft
l^ii^
■^ „♦
^
»«
•*-
>. r
^
Fig. 222. Two additional small-chromosomed species of horsetail for comparison of sizes and shapes
with Figs. 213 and 215. x 1000. a. Equisetum limosum L. diakinesis in a Feulgen squash. For
explanatory diagram see Fig. 223. n- 107-108. b. E. palustre L. diakinesis in acetocarmine showing
more usual size appearance with this technique in contrast to the exceptional degree of
enlargement shown in Fig. 2i8Z». c. E. limosum, metaphase for comparison with the large-
chromosomed species shown in Fig. 213^.
species or subspecies, and it was with considerable surprise that unmistakable cytological
signs of hybridity were discovered in it.
It will be convenient to consider E. litorale first. This plant (Fig. 224), first found in
central Europe, was regarded as a probable hybrid between E. arvense and E. limosum
by Milde in 1867 on the ground both of its intermediate morphology and its abortive
224
THE GENUS EQUISETUM
spores, and although Milde began to doubt this diagnosis in later life owing to the in-
crease in the number of recorded localities, his original reasons are still valid. The
abortive spores in themselves suggest hybridity even more clearly to us than to a
botanist in Milde's day, and the morphology of the plant is not only intermediate be-
tween the two suspected parents but ranges rather widely between the two extremes in
different localities, thus suggesting that it is either re-formed from time to time from
different strains or that a limited degree of fertility exists in it and therefore some
genetical segregation.
I am indebted to Dr Praeger for supplying me with the distribution map shown in
'y^
J*>r
£. /imosum n = 108
Fig. 223. Explanatory diagram to Fig. 222a.
Fig. 225, which indicates that E. litorale is scattered widely over Ireland with a frequency
which bears some relation to the closeness of sampling; the abundance of localises in the
Belfast area, for example, being probably diagnostic of the alertness of the Belfast
Natural History Society rather than denoting an actual preference for this region.
Living material from two sources was sent to me. In one, which came from Galway Bay,
partially expanded cones much resembling E. arvense but with wholly aborted spores
were too old to investigate. The second specimen came from eastern Ireland and grew
for many years in cultivation, but gave only sterile shoots much resembling E. arvense
in appearance. A visit to the wild locality of this material in June 1938 with Dr Praeger
showed it (Fig. 224) to be in habitat an aquatic plant as is E. limosum, though with
a solid branched stem like E. arvense. The cones were, however, borne exclusively on
the ends of the current green shoots as in E. limosum. From these, successful fixings
MPC 22^ ^•'
Fig. 224. Silhouette of a pressed fertile shoot collected in June of the Irish material
of Equisetum litorale Kuhlw. Natural size.
Hyhrid Equisetunv
in,
IRELAND
^^ = E- litoraLe
• ~ E. trachyociorv
~ E. Moorei
Fig. 225. Map of the distribution of known localities of hybrid horsetails in Ireland,
from information kindly supplied by Dr Praeger.
227
15-2
THE GENUS EQUISETUM
were obtained, and the section shown in Fig. 226 at once gives confirmation of the
diagnosis of hybridity and a reason for the abortion of the spores. Fig. 227a, from an
acetocarmine preparation, adds details of the pairing and shows again the large number
of univalents.
In the hope of synthesizing E. litorale, prothalli of the two parent species were grown
in 1939, and insemination from young males of £. limosum into old females of £. arvense
was watched under the microscope. Unfortunately, the difficulties of culture were not
successfully surmounted for the later stages and all the inseminated prothalli died from
fungus attack. If these cultural difficulties could be overcome the synthesis oi E. litorale
ought to be possible.
E. trachyodon A.Br, is known in Floras as a very rare European plant in which defective
spores were detected as long ago as 1864 by Duval-Jouve. It is best known from the
Fig. 226. Meiosis in Equisetum litorale Kuhlw. in a section
showing lagging unpaired chromosomes, x 1000.
Rhine valley and from Ireland, although single stations are now on record for a few
other countries, e.g. Scotland and Latvia (Kupfer 1929). In contrast to E. litorale,
E. trachyodon is very uniform morphologically wherever it is found, although Milde was
able to detect a few minor differences between the Irish and Rhineland specimens.
As in E. litorale the spores of E. trachyodon are completely aborted, and the means for
its very wide distribution are at present quite unknown.
As in the previous case, my material of £. trachyodon came from Ireland. I visited an
east Irish locahty with Dr Praeger in June 1938, and fixed wild cones, but this particular
plant cones also readily in cultivation and abundant material is easily obtained. The
principal results are perhaps sufficiently shown by Figs. 227 <: and d, representing
diakinesis and metaphase (or anaphase) respectively. The almost complete absence of
any sign of pairing is the most characteristic feature, although a solitary bivalent can be
seen on the spindle in Fig. 22 7^.
Failure of pairing on this scale is the obvious explanation for abortion of spores, and
it is not, in fact, more extreme here than in some other well-authenticated wild hybrids,
notably some of the triploid ferns such as the Polypodium of Fig. 139. It should,
228
1
^^ *-->
*<^' V
V ;. . .'
"^-t.
^ "
*f« .1 V ♦*, i^«»
• I ♦^ . *
.,f
« ■ 1*
'^'
Fig. 227. Meiosis in three hybrid horsetails in permanent acetocarmine mounts, x 1000. a. Equisetum
litorale Kuhlw. first meiotic metaphase showing pairs and univalents, b. E. Moorei Newm. the
same showing pairs, univalents and multivalents, c. E. trachyodon A.Br, diakinesis showdng complete
failure of pairing, d. The same at anaphase of the first meiotic division showing one pair on the
equator and the remaining unpaired chromosomes distributing themselves at random to the poles.
229
THE GENUS EQUISETUM
however, be noted that complete failure of pairing can sometimes be produced by other
means, either metaboHcally or owing to an asynaptic mutation. If the latter were the
case here, however, it is difficult to see why an asynaptic mutant should be the only
representative of a species to survive and positive evidence for the former is wholly
lacking. Indeed, in an attempt to alter the grade of chromosome pairing experimentally,
a piece of the plant was transferred to a hot greenhouse and maintained at tropical
1 #
Fig. 228. Spiral structure at anaphase of the first meiotic division
in Equisetum trachyodon A.Br, x 2000.
temperatures (70° F.) for over two years without the slightest effect. A hybrid origin
therefore remains the most probable explanation for it, and the most probable parents
in that case are E. variegatum x E. hiemale.*
In spite of the absence of experimental proof of the suggested parentage of £■. trachyo-
don there would probably be little reason to look elsewhere for a mode of origin but for
the circumstance that the third case of an apparent hybrid, E. Moorei, seems to relate
to the same two species, although it is itself quite unlike E. trachyodon in habitat and
appearance. E. Moorei is a maritime plant of sandy soil found for about 100 miles along
* A relationship with the south European E. ramosissimum has sometimes been suggested, which I was
at first inclined to disregard since this species was thought not to be British. I have however recently been
informed by Mr. A. H. G. Alston of the British Museum that an authentic specimen of is. ramosissimum
has recently been found in Norfolk and it may therefore perhaps also have existed in Ireland in former
times. If this were so it would greatly assist the problem of finding suitable parents for both E. trachyodon
and E. Moorei in that country.
230
THE GENUS EQUISETUM
the coast of Wexford {E. trachyodon is a plant of river margins). It has relatively stout
erect green shoots more like those of E. hiemale than of E. variegatum, though with char-
acteristic details of the surface markings of its own. It cones in August. Fig. 227^ shows
metaphase i in E. Moorei. The very irregular pairing, including multivalents and uni-
valents, is very striking. This, together with abortive spores, is a clear indication of
hybrid origin, and though speculation might be possible to explain the coexistence of
both E. Moorei and E. trachyodon in Ireland, it is more profitable to defer this until a
hybrid of appropriate kind has been synthesized. Until this has been done it is perhaps
best to confine our present conclusions to the statement that E. Moorei is not a good
species in the usual sense of the word and to commend it to Irish botanists for closer
scrutiny.
The detection of three species hybrids among little more than a dozen representatives
of the genus is a surprisingly large number, especially when the rarity of prothalU is
remembered, and it suggests fairly clearly that speciation can still occur. The complete
absence of polyploidy is, however, noteworthy, and the extreme uniformity in chromo-
some number found throughout the genus is also in striking contrast to the behaviour
of ferns. At the same time a haploid number as high as 108 cannot be thought of as
primitive, though we are never likely to know by what stages or from what simpler
state it was evolved. As we find them now, the Horsetails give the impression of being
a very ancient and a very stable group, still able to make new species by genetical means
though doing so only slowly, but long out of the habit of giving rise to new generic types.
The very stereotyped morphology is perhaps not so much primitive as speciahzed upon
an archaic pattern, and the general cytological condition is perhaps also best interpreted
as ancient rather than simple.
SUMMARY
Mitosis and meiosis in all the European species oi Equisetum and of one American species
have been seen. In all, the haploid chromosome number is either exactly or approxi-
mately 108, though a slight possibihty remains of a minor numerical difference between
species of the two subgenera, Eu-equisetum and Hippochaete. There is a diflference of
chromosome size between these subgenera, the latter having the larger chromosomes.
Three Irish forms, namely, Equisetum litorale, E. Moorei and E. trachyodon, have been
shown to have the meiotic behaviour of hybrids. Spiral structure of chromosomes has
been demonstrated in E. trachyodon. The list of species and hybrids examined is :
Subgenus Eu-equisetum
Species
Source
n
E. arvense L.
Manchester
Probably 108
E. maximum Lam.
»>
E. sylvaticum L.
>>
E. praterise Ehrh.
Hort.
E. palustre L.
Manchester
E. limosum L.
3>
E. litorale Kuhlw.
Ireland
Irregular meiosis
231
THE GENUS EQUISETUM
Subgenus Hippochaete
\
Species
Source
E. ramosissimum Desf.
Italy
Probably io8
E. hiemale L.
Durham
E. rohustum A.Br.
Hort.
E. variegatum Schleich.
Southport
Large form
Dublin
E. scirpoides Michx.
Norway
E. trachyodon A.Br.
Ireland
Irregular meiosis
E. Aloorei Newm.
Ireland
)j
232
CHAPTER 14
THE PSILOTALES
The Psilotales are a microphyllous group without close relatives and without known
ancestry. They are microphyllous in the sense that they are certainly not megaphyllous
like the ferns, though it is an open question whether in fact they possess leaves at all, the
curious little appendages borne on the aerial portions of their stems being so unlike
leaves in the ordinary sense that they are perhaps not of this nature. Psilotum and
Tmesipteris are the only known genera, each containing only one, or at most two or three
species, none of which is native to Europe or nearer to our shores than the tropics and
the southern hemisphere. Nevertheless, so important are they in all morphological dis-
cussion of the Pteridophyta that they cannot be omitted. In their complete absence of
roots and in other relatively simple features they are now generally regarded as in all
probabihty the most primitive of living vascular plants, and if any relationship can be
established with other members of the Pteridophyta it is Hkely to be with the long-
extinct Psilophytales, themselves the simplest vascular plants known to science, rather
than with any more recent group.
Fortunately, though not native to Europe, examples of both genera are available in
botanic gardens and, in addition, I was in the uniquely favourable position of having
been entrusted in 1939 with the cytological examination of both sporophytes and
gametophytes collected in New Zealand by the late Dr HoUoway of the University of
Otago and described by him in the Annals of 'Botany of 1938 and 1939. There were
special peculiarities about Dr Holloway's prothalli which made a cytological study of
them desirable, and the cytological findings were published in detail in 1942 (Manton)
to preserve them from risk of loss by enemy action. The observadons recorded in that
context are still the most important contribution that I personally have to make to know-
ledge of this group, but through the kindness of Professor H. N. Barber of Tasmania, who
has communicated privately some additional observations from Austrahan material with
permission to quote them, a few additional facts can now be supplied. Since I understand
that Professor Barber is also himself engaged on further study of both genera on native
Austrahan material, the account to be given below need only be regarded as an interim
statement, of value in the present connexion for comparative purposes with the other
great groups, and as neither final nor exhaustive in itself.
Members of the genus Psilotum are found in the tropics right round the globe, though
they are familiar objects in most botanic gardens since they are fairly easy to grow and,
in addition, reproduce themselves by bulbils wliich are very easily transported. They
are therefore Uable to spring up unexpectedly as weeds in tropical glasshouses where pot
plants are grown, and may have been repeatedly introduced to Europe along with other
plants. This is partly the reason why a place of origin can rarely if ever be assigned to
botanic garden material, and even in Japan, the only country in which they appear ever
to have been extensively grown for horticultural purposes (Okabe, 1929), it cannot be
233
i
i
Fig. 229. Shoot of tetraploid Psilotum nudum (L.) Beauv. (P. triquetrum Swartz) of unknown wild origin,
grown at Kew. From a herbarium specimen from which the sporangia fell off as a result of drymg.
Natural size.
THE PSILOTALES
certainly known that all strains are of Japanese
origin. For this reason the wild material to be
described below is of particular interest.
Of recognized taxonomic species there are
two: P. Jiaccidum Wall., Fig. 230, a rather un-
common pendulous form with flattened stems
and P. nudum (L.) Beauv. {P. triquetrum Swartz),
Fig. 229, with stiff, erect stems, which is much
more frequent. To this species Dr HoUoway's
prothalli belonged, and since the prothalli
were both the starting point and the centre of
interest of my personal connexion with the
whole group, it will perhaps be appropriate
to start with them.
The sexual generations of the Psilotales have
so far only been found in the southern hemis-
phere and by very few observers. The first
gametophytes of Psilotum were detected near
Sydney, Australia, by Darnell Smith and by
Lawson, both of whom published notes about
them in 191 7, though not all stages required
for a complete description of the life history
were present. They were apparently not found
again until Holloway did so on Rangitoto
Island, Auckland, New Zealand, where
numerous examples were obtained, many of
them bearing young plants, on each of two
visits made with an interval of seven years
between. These prothalli were the basis of
HoUoway's paper of 1938 and of my own paper
of 1942. In the meanwhile Holloway had also
completed the life history of Tmesipteris by
finding prothalli and young plants of this
genus also in New Zealand (Holloway, 191 7,
192 1 ). By the activities of this one worker the
main developmental facts for both genera of
the Psilotales have therefore been elucidated,
though the cytological interpretation is still
somewhat imperfect, as will shortly be seen.
Fig. 230. Part of a diploid shoot of the pendulous
Psilotum Jiaccidum Wall, of unknown wild origin, grown
at Kew, from a herbarium specimen. Natural size.
235
THE PSILOTALES
The prothalli of both Psilotum (Fig. 231) and Tmesipteris are small cylindrical struc-
tures of a few millimetres or perhaps centimetres in length in the largest specimens,
living underground with the aid of a symbiotic fungus. Branching is dichotomous and
growth is by an apical cell as in the subterranean rhizomes of the sporophyte, which they
indeed closely resemble except for the numerous sex organs which they bear over their
surface. An additional point of resemblance met with in the Rangitoto material of
Psilotum was the presence of vascular tissue. This was found only in the largest prothalli
and it varied in extent. In the simplest cases the centre of the prothallus was occupied
by a group of elongated thin-walled cells free from fungus, as may be seen in Fig.
232 fl. In other cases an endodermis could be seen surrounding such cells, and
Fig. 231. The vascular prothalli of Psilotum nudum from Rangitoto Island (see text), a. Intact prothalli
preserved in 70% alcohol, x 4. Inset natural size. b. A dividing cell from a prothallus from
this locality, after Manton (1942). x 1000.
in yet other cases the central strand contained actual tracheids with lignified walls.
Some of these can be seen in the longitudinal view photographed by Mr Ashby from
one of HoUoway's original preparations reproduced in Fig. 233. The prothallial
vascular strand was liable to be interrupted at intervals by the invasion of the entire
central tissue by the symbiotic fungus, in which case the central cells remained un-
differentiated, and in the smaller prothalli, which formed the bulk of the collection,
they were quite absent. They were, however, obtained from different parts of the island
on each of Dr HoUoway's two visits, and could therefore not be dismissed as local mal-
formations, and it was their presence which prompted Dr Holloway to seek for a cyto-
logical investigation.
The results of such an investigation carried out on material supphed by Dr Holloway
in alcohol after fixation in acetic-alcohol were that all sizes of prothalH from Rangitoto
Island were diploid, and that the sporophytes on the island to which they were no
doubt related were tetraploids. In addition to being tetraploids, which would not in
itself explain the presence of vascular tissue in the prothalh, a very characteristic and
236
1
i
Fig. 232. Cross-sections stained in haematoxylin and Bismark brown to show a comparison between
a prothallus and a rhizome in Psilotum nudum (L.) Beauv. (P. triquetrum Swartz). x 40. a. A diploid
prothallus from Rangitoto Island showing central area free from the endophytic fungus though
this specimen is without actual tracheids. Note projecting antheridia. b. Transverse section
of a tetraploid rhizome from Malay showing fungus and central vascular tissue.
'I'V'
y-~
Fig. 233. Vascular prothallus from Rangitoto Island to show detail of central conducting strand.
a. Longitudinal section. b. Transverse section. Both x 100.
THE PSILOTALES
unusual malformation of the spindle was found at meiosis in many of the spore mother
cells, which resulted in tripolar or quadripolar figures at the first division (Fig. 234c) and
from six to eight nuclei at the end of the second. This abnormality was thought to be
metabolic rather than cytological in origin, since it is not a normal attribute of tetra-
ploidy as such, but it confuses the interpretation of the prothallial structure by intro-
ducing a possible source of genetical abnormality into their make-up. This risk may be
A:
t
J ' ♦
'•^•t.«f
Fig. 234. Tetraploid sporophyte of Psilotum nudum (L.) Beauv. (P. triquetrum Swartz) all after Manton
(1942). a. Tip of a fertile twig of a sporophyte from Rangitoto Island decolorized in alcohol, x 2.
b. Mitosis in a tetraploid rhizome from Malay in which a chromosome count ('over 200') was
made, x 1000. c. Group of four mother cells showing in-egular meiosis with a tripolar spindle
from the Rangitoto material, x 500.
thought of as more apparent than real, since spores as irregular as those observed are
most unlikely to have been viable, and reproduction of the species could easily be con-
fined to the normal mother cells which were also present. The fact of polyploidy, how-
ever, at once disclosed the need for further investigation of both generations on wild
material.
It is at this point that Professor Barber's recent observations are of special interest. He
reports that material growing wild near Sydney, Australia, is also tetraploid. This means
that Darnell Smith and Lawson's (1917) source of prothalli is hkely to be of the same
general type as that on Rangitoto Island and a true haploid is still to seek. Further,
Professor Barber made the interesting observation that meiotic irregularities involving
tripolar spindles were also encountered by him at Sydney on wild material fixed after a
spell of unusually cold weather, but that the same material after transfer to the laboratory,
238
THE PSILOTALES
where it was merely kept in water for a spell of some weeks, was still in meiosis at the end
of the time but showed perfectly normal spindles. This confirms the suggestion made in
relation to Rangitoto that the meiotic irregularities found there are metabolic in origin.
Tetraploid sporophytes are thus now known from New Zealand and Australia; they
may be suspected to be present in parts of Japan from the work of Okabe (1929), and
a rhizome count reported from Malay (Manton, 1942) was also of this nature. The only
certain diploid sporophyte so far referable to an exact locality is one from Ceylon
(Manton, 1942). It is obvious, however, that very large tracts of the earth's surface are
entirely unexplored, and it is still too soon to know whether the diploid or the tetraploid
sporophytes will have the wider distribution.
'¥ V
1
t H'
\
1
4 ^
j||a|
Fig. 235. Meiosis in tetraploid Psilotum nudum (L.) Beauv. (P. triquetrum Swartz) from Kew, permanent
acetocarmine. x 1000. From the specimen of Fig. 229, after Manton (1942).
From what has been said it will be obvious that in Psilotum the fact of polyploidy is of
far greater interest than the actual details of the chromosome numbers, and for this
reason it has been discussed first. It now remains to add the numerical details as far as
these are known, which, unfortunately, is still only imperfectly. My own information is
still exactly as in 1942. The only diploid sporophyte available to me alive was a plant
of P.Jlaccidum, of unknown wild origin, growing at Kew (Fig. 230, p. 235). In this there
are not less than 52, nor more than 54, chromosome pairs at meiosis (Figs. 236 a, 237),
though I was unable to decide between these two numbers. This is the only record of
P. flaccidum so far available, but in the best modern study of P. nudum {P. triquetrum)
known to me (Okabe, 1929) the haploid number for horticultural strains in Japan is
given as 52. This number is therefore undoubtedly very near to the truth. Fig. 235
shows meiosis in a tetraploid sporophyte, also of unknown wild origin, growing at Kew in
239
THE PSILOTALES
which the doubled number of rather smaller chromosomes is visible. Since it is profitless
to attempt exact enumeration in a tetraploid without having a diploid of the same species
for comparison, I have made no further attempts at greater precision. There is, however,
a strong suggestion in Fig. 235 of the presence of some multivalents, and Professor
Barber informs me that the same may be seen in Australian tetraploids. It is therefore
possible that in this particular species we are dealing with an autopolyploid series.
This is as far as knowledge oi Psilotum can at present be carried, and there is clearly
scope for much further work by anyone with direct access to wild material. The same
4 ^m
1
k.
^■:
Fig. 236. Meiotic metaphase and anaphase In Psilotumfiaccidum Wall., permanent acetocarmine. x 1 000.
a. After Manton (1942), explanatory diagram in Fig. 237. b. Showing traces of spiral structure.
is also true of Tmesipteris (Fig. 238). This has been available to me only in Botanic
Garden material growing on the stem of a tree fern at Glasnevin in Dublin and believed,
though not certainly known, to have come from Tasmania. Since Tasmania is now the
centre of Professor Barber's activities, who may be expected shortly to unravel the whole
situation, description of my results can be brief. Fig. 239a shows a somatic division in
a tapetal cell, and Fig. 239^ is a mother cell at the first meiotic division. The number
of chromosomes is horrifying, and I do not propose to attempt an exact enumeration.
It is sufficient to state (Fig. 240) that the sporophytic number is between 400 and 500,
and that the corresponding gametophytic number is somewhat over 200. That this is
also a case of polyploidy is known from an earlier description of c. 100 at meiosis by
Yeates (1925) and from Professor Barber himself, who has detected (personal com-
munication) two chromosome numbers in Australian material, of which 'somewhat over
200' is the higher. With this intimation that polyploidy exists in both genera, we may
therefore safely leave the matter in Professor Barber's hands.
Two things may, however, perhaps be added by way of comment. Both Psilotum
Haccidum and Tmesipteris display the same type of curious chromosome shape as that
already seen in Equisetum and referable to the same cause, namely, a characteristic
240
THE PSILOTALES
laxity of the spiral structure, traces of which are nevertheless detectable at anaphase in
the former species (Fig. 236). These appearances, which will be met with again in
Lycopodium and Isoetes, may be of little phyletic significance; on the other hand, they may
perhaps add one slight contribution to the other reasons for believing these various
genera to be related.
Secondly, with regard to the details of chromosome number, it is much to be desired
that the demonstration of the basic haploid for the group should be made clear with
complete finality, since in the present state of knowledge comparisons are suggested
Zorjr
Psi/otum n = 52-4
Fig. 237. Explanatory^ diagram to Fig. 236a
from Manton (1942). x 1500.
Fig. 238. Small shoot of Tmesipteris tannensis (Spreng.)
Bernh. of unknown wild origin from a live specimen
grown in the Botanic Garden at Glasnevin, Dublin.
Natural size.
which, if based on erroneous information, may be very misleading. If the lowest
gametic number were indeed to be 54, it is tempting to draw a comparison not only with
the 108 of Equisetum but also with n = g o{ Selaginella which will be seen in the next
chapter. It is conceivable that all these high numbers are the end-products of very
ancient polyploid series which relate back to simple beginnings even in those groups such
as Equisetum, in which no direct traces of polyploidy at present remain. Further discus-
sion of this is not possible without facts of irreproachable accvu"acy to go upon; the
matter is, however, worth mentioning here in order to act perhaps as a spur to a favoured
observer who, with wild material accessible to him, may perhaps be tempted thereby to
make the very considerable effort required to remove our present doubts.
SUMMARY
Summing up the facts for the Psilotaceae, even in their present imperfect state, it has
now been shown that polyploidy exists in each of the two living genera of Psilotum and
Tmesipteris, the lowest number now known at the base of the series being of the order of
50 odd in Psilotum and 100 odd in Tmesipteris. The fullest information at present
16
MPC
241
^
\ ' , r>"
.<v. ■ <
^"^ ■■-■ ..t
V
f' ■
i
#
'I
J
-44* * V.
V,
I
Fig. 239. Cytology of the Dublin specimen of Tmesipteris tannensis (Spreng.) Bernh. Permanent aceto-
carmine. x 1000. a. Mitotic metaphase in a tapetal cell with 2?j = over 400. i. Meiosis in the
same with n = over 200.
242
THE PSILOTALES
available refers to tetraploid Psilotum which has now been obtained from New Zealand,
Australia, Malay, and perhaps Japan, while a corresponding diploid has only so far been
found in Ceylon, though it may easily be present in other parts of the world. A char-
I
w.
ir^
Tmesipferis 2n =■ over 400
Fig. 240. Explanatory diagram to Fig. 239a. x looo.
acteristic aberration of the spindle reported from Japan, New Zealand and Australia has
been shown by Professor Barber of Tasmania to be of metabolic origin in unfavourable
environmental conditions. The causes of the presence of anomalous vascular tissue in
the diploid gametophytes described by the late Dr Holloway from New Zealand are still
uncertain, and a genuine haploid prothallus is still undiscovered in this genus. Further
study of both genera and of all species in them is much to be desired.
243
16-
CHAPTER 15
THE LYCOPODS (CLUBMOSSES
The next great group, and the one to which the name microphyllous particularly
applies, albeit with reservations in the case oilsoetes, is that of the true Lycopods. These
consist of three exceedingly dissimilar genera, Lycopodium, Isoetes and Selaginella, all of
which are cosmopohtan and of one additional monotypic genus confined to Australia
and New Zealand, Phylloglossum. Further reference to Phylloglossum will, for the moment,
be omitted, since the centre of interest throughout this book is among wild European
plants wherever possible. The contents of this
chapter will, therefore, be confined to the
British representatives of Lycopodium, Isoetes
and Selaginella, slightly supplemented, in the
case of Selaginella, by examination of two non-
British wild European species.
The British Lycopods comprise only nine
species, namely, three of Isoetes, five of Lyco-
podium and one Selaginella. All are small in
stature and quite insignificant components
of the vegetation, but in structure, life history,
ecology and past history they combine more
points of interest, and raise more fundamental
botanical problems than can be found at one
time in any other group. They are, moreover,
each and all sufficiently localized and distinc-
tive to constitute landmarks in the recollection
of every field botanist or nature lover who has
made their acquaintance by his own exertions.
Taking the genus Lycopodium first as per-
haps the best known to the average field
naturalist, we have five British species, all of
them to be found among mountains and most
of them confined to such regions. L. clavatum
(Fig. 241), Hhe Clubmoss' par excellence, is
also to be met with on heaths, where its creeping branched stem, densely clothed with
little leaves and rooted at intervals, may cover many yards of ground. Very similar in
habit, though more restricted in range, is L. annotinum L. (Fig. 251), locally abundant in
the Scottish Highlands but very rare in England, the best known locality being one hillside
in the Lake District where I suspect that it is not always fertile (see below). L. alpinum L.
is somewhat more specialized in structure, having a subterranean, colourless, creeping
stem from which little tufts of aerial branches, closely pressed to the ground, arise at
Fig. 241. ^Whouciit oi Lycopodium clavatum
L. from Borrowdale. Natural size.
244
THE LYCOPODS (CLUBMOSSES)
intervals. It occurs characteristically on mountains above the tree line. L. inundatum L.
is probably the most difficult species to detect, since the length of its stem is to be
measured in inches and not feet, although its leaves and solitary cone are in themselves
as large as in the others. It is scattered over the country at various altitudes but always
in boggy places and often in quite small colonies.
L. Selago L. completes the list. This very conspicuous little plant, with the habit of an
erect dwarf bush covered with spreading spiny leaves, is the most easily found of all our
British species at an appropriate altitude, which is usually slightly lower than that
required by L. alpinum. It differs from all the others in ways which appear to be
morphologically primitive. The radially symmetrical erect stem and the absence of
specialized cones are two of the more important points, and in these respects its nearest
relatives among living species are to be found in the tropics, e.g. L. squarrosum; though if
fossils are also related, a point which must not be too readily assumed, L. Selago seems to
show more features of (perhaps superficial) resemblance to very ancient types such as
Drepanophycus (Devonian) or Baragwanathia (Silurian) than do any of the other species
listed. The interest of the relatively primitive construction in Lycopodium Selago is
somewhat enhanced by a geographical distribution which, at present, is virtually
worldwide, and by an abundance of individuals which is made possible by a very
characteristic vegetative reproductive mechanism. Detachable bulbils are borne in
place of some of the leaves on zones of the stem, alternating with zones in which the
leaves bear axillary sporangia, and these bulbils germinate very readily to produce new
plants.
The sexual generation of the genus Lycopodium was for long a matter of speculation,
for it is exceedingly difficult to detect in nature, and the spores, though produced in
such abundance as to be sold commercially as 'Lycopodium powder', will only germinate
under very special conditions and even then only after a lapse of several years. The early
germination stages of L. inundatum were, however, seen in 1858 by de Bary, and adult
prothalli of the same species were found wild in 1887 by Goebel in Germany. Prothalli
and young plants of L. annotinum were found in Switzerland by Fankhauser in 1873 ^^*^
again in Germany by Bruchmann in 1884, after which this last indefatigable and gifted
observer proceeded to discover the prothalli of all the other European species (Bruch-
mann, 1898) and at a later stage germinated their spores (Bruchmann, 1910). All these
specimens were of continental origin and most of them were from the Thuringerwald
and the Harz Mountains where, to quote Bruchmann, they are not so much rare as very
local in their occurrence. Prothalli have, however, also been seen in Great Britain by
W. H. Lang, who discovered those of L. clavatum in Scotland in 1899 and of L. Selago on
two occasions subsequently (Lang, personal communication). I am indebted to Pro-
fessor Lang for permission to reproduce an old photograph taken at the time of the dis-
covery of the prothalli and young plants of L. clavatum (Fig. 242) which will serve to
illustrate the type of structure. The gametophytes are colourless subterranean organ-
isms living saprophytically with the aid of an endophytic fungus as in Psilotum (see pre-
vious chapter), but with a characteristic shape which differs considerably from those of
that genus. There is therefore no risk of confusion with other organisms even if the
identity were not attested by the presence of young sporophytes. In practice, however,
245
THE LYCOPODS (CLUBMOSSES)
it is most unlikely that prothalli would be found at all without the index of juvenile
sporophytes to draw the searcher's attention to a suitable spot.
These facts regarding the discovery of the sexual generation have been given in some
detail, partly for their own interest in giving a picture of the genus, but also in relation
to the cytological observations on Lycopodium Selago which will be described below. As
far as field observations go that species differs in no essential way from the others.
Gametophytes are equally rare in all, being
apparently produced sporadically in response
to locally favourable conditions, and the main
colonization of territory is carried out by vegeta-
tive growth in the creeping forms, or by bulbils
in L. Selago.
Difficult as it is to grow the prothalli and spores
under artificial conditions, the culture of the
sporophytes is scarcely less so with the one excep-
tion of L. Selago, which will grow readily in pot
culture. I have also myself succeeded in keeping
L. annotinum alive for several years, but in no case
are cones normally produced in culture in the
British species. This may suggest that they are
not perhaps ideally suited to cytological study,
and it may be said without fear of contradiction
that for this purpose they are the most awkward
genus of Pteridophytes in the whole of the British
flora. It is not merely that the appropriate
seasons for their study are short and the plants
relatively inaccessible, but fixation presents acute
difficulties which, added to the extremely peculiar
shapes of the chromosomes and other attributes
which will shortly be mentioned, may make even a successful preparation very difficult
to interpret.
The easiest species to study, however, somewhat surprisingly, and perhaps fortuitously,
proved to be L. inundatum. This rather uncommon species was sent to me by post from
Aviemore (Scotland), in perfect condition, in July 1936. It had been packed tightly in
a tin amongst moist moss, and was still in full meiosis when received. It gave the pre-
paration shown in Figs. 243 and 246 a, in which the antenna-like form of the chromo-
somes is strikingly displayed. Their number is, however, not in doubt : n = 78.
The next species to give a result was L. clavatum. This is rather later than most in
maturing its spores, and the end of July is more favourable than earlier in the month,
both in Scotland and in the Lake District. At the beginning of July, at which season my
own material was obtained in the Lake District, meiosis can only be found in the largest
cones available, and farther north, in Scotland, this date is definitely too soon. Once
material of the right age has been obtained the cytological observations are fairly straight-
forward. Fig. 246 fif shows diakinesis sufficiently well spread to enable one to dispense with
246
Fig. 242. Lycopodium clavatum L. Young
sexually produced plant (right) and a
prothallus (left). Natural size. From a
photograph kindly supplied by Prof.
Lang (cf. Lang, 1899).
THE LYCOPODS (CLUBMOSSES)
a diagram; there are 34 pairs of chromosomes. Fig. 246^ (at a higher magnification)
shows the first meiotic metaphase for which a diagram is supphed in Fig. 244, again
showing n = 34. Confirmation of this number to the extent which is possible from sec-
tions has been made on roots of both Swiss and British material, in each of which 2n = 68
as nearly as can be determined.
Z Ljcopod/um inuna/a/^um n - 78
Fig. 243. Explanatory diagram to Fig. 246a. x 1500.
L. annotinum, the next species, is rare in Britain as already pointed out. My material
of it is less complete than could be wished since the Scottish localities were inaccessible
during the war and in its only English station* it appears to be receding for chmatic
reasons. It was not fertile when I visited the Lake District in June 1939, and my
* I am informed by my colleague, Dr Sledge, that the species has been found in one other place,
in Yorkshire.
247
THE LYCOPODS (CLUBMOSSES)
observations on British material are therefore confined to roots. Fortunately, these
were unusually clear and they have been supplemented by roots fi-om a specimen
fixed in Switzerland (Fig. 251) and also by a few cells
in meiosis obtained at Storlien in Sweden in July
1948. The meiotic material is somewhat scanty and
requires confirmation. One cell, however, is illustrated
in Figs. 247a and 248, and the chromosome number
appears to be ^ = 34 as in L. clavatum. This is in close
agreement with the evidence from roots (Fig. 247/)) in
Britain in which also 2n = probably 68. It is therefore
certain that L. annotinum both in Britain and on the
Continent is very similar to L. clavatum and is probably
identical cytologically with that species.
This cannot be said of the other two species. L. alpinum,
in spite of the high altitude of its habitats, matures its
cones some weeks before those of L. clavatum in the same
district, and June would be the best month to seek for it.
In the first week of July 1944, only a few residual cones
Z. c/oi^atum n ^34
Fig. 244. Explanatory diagram
to Fig. 246^. X 2000.
I
+
AJ X
L. a/pinum n ^ 24 - 5
Fig. 245. Explanatory diagrams to Figs. 2466 and c, for description see text, x 2000.
were still young enough to be used, while L. clavatum at the same date had scarcely
begun meiosis. Figs. 245 and 246 /> and c show two stages in spore mother cells of
L. alpinum. At the metaphase of the second meiotic division (Figs. 245a, 246<:)
there appear to be 25 split chromosomes, of which one half-chromosome in the
middle of the field appears to have become separated rather widely from its fellow in
the making of the preparation. This specimen is of importance because, unsuited as
248
Fig. 246. Meiosis in British species of Lycopodium, permanent acetocarmine. All except e, x 1000.
a. L. inundatum L., first meiotic metaphase. n= 78. b. The same in L. alpimim L. For explanatory
diagram see Fig. 245 i. c. The same, at the second meiotic metaphase. For explanatory diagram
see Fig. 245 a. d. Diakinesis in L. clavatum L. « = 34. e. The same at metaphase. x 1500. For
explanatory diagram see Fig. 244.
249
THE LYCOPODS (CLUBMOSSES)
«
^ J
*
1
^
€
td^
'#
#
Jr
*i5; *
*
♦^
I
4
a b
Fig. 247. Chromosomes of Lycopodium annotinum L. x 1000. a. Meiosis in a Swedish specimen in
balsam after acetocarmine. For explanatory diagram see Fig. 248. n = probably 34. b. Root-tip
section showing mitosis in a British specimen, for comparison of chromosome size with other plants.
-^
Z. annof-inum n = '^■^
Fig. 248. Explanatory diagram to Fig. 247 a.
is the second division for accurate counting in almost every member of the Pteridophyta,
the first division in this particular species seems to be worse. Diakinesis is unusable
owing to diffuseness of chromosome outline, which is more extreme in L. alpinum than in
L. davatum, though it is also apparent (Fig. 246^) in that species. At metaphase (Fig.
2466), on the other hand, the despirahzation of the chromosomes is so extreme that the
task of disentanghng them is almost insuperable. This figure is an unusually successful
attempt at doing so, which at first counting gave 23 or 24 pairs. It is, however, just
possible that the cell is incomplete. The chromosome number must therefore be left
uncertain as not less than 24 nor more than 25.
L. Selago remains, and here the cytologist's troubles reach a climax in spite of the ease
of cultivation and other advantages which one might expect would facilitate the task. In
actual fact this species is, in my experience, the worst cytological object that I have ever
encountered, and in the unequal contest between cytologist and plant, the plant has in
this case so far won handsomely. The reason is that fixation of roots is virtually hopeless
by all the older methods, and at meiosis, even with modern methods, the combination
of high chromosome number with extreme irregularity of pairing produces a most
intractable situation. Simple chromosome enumeration becomes virtually impossible at
250
THE LYCOPODS (CLUBMOSSES)
O
a b d
Fig- 249. Meiosis in Z^co/)o</zMm6'e/a^oL., permanent acetocarmine. a. Polar view of the first metaphase.
X 1000. Forexplanatory diagram see Fig. 250. b. Side view of the first anaphase showing numerous
lagging univalents, x 1000. c. Side view of the first metaphase showing pairs and lagging univalents.
X 500. d. Side view of the second anaphase showing lagging half-chromosomes derived from split
univalents, x 500.
any stage, and, without accurate knowledge of
chromosome number, detailed analysis of meiosis
cannot be carried out. It is obvious that nothing
less than a prolonged special study and probably
the application of new technical methods will
be needed to break this deadlock, and only an
approximate result can be given here. Demon-
stration of the qualitative side of irregular
pairing is, however, not difficult. Fig. 249c
shows the first meiotic metaphase, at a low
magnification, in a squash preparation in which
a cloud of univalents are conspicuous objects.
Laggards at anaphase, i.e. the univalents which
are splitting, are equally conspicuous in Fig. 249 b,
and they appear again at the end of the second
meiotic division in Fig. 249 a'. A fuller demon-
stration of the qualitative side of pairing is
scarcely required, and it should perhaps also be
pointed out that these figures do not represent
one plant at one season but are from several
places in several years. Fig. 249/* being from a
Scotch plant in 1936, Fig. 249 c and d from a
Welsh plant in 1938 and Fig. 249 a from a Lake
District plant in 1944. Irregular pairing must, therefore, be recognized as a charac
teristic feature of Z-. Selago over much, though not necessarily all, of Great Britain.
O
O
0
to
o
Z. Selago
Fig. 250. Explanatory diagram to Fig. 2495.
X 2000. Paired chromosomes in black,
univalents in outline.
(]
251
THE LYCOPODS (CLUBMOSSES)
Compared with this fact the interest of the actual chromosome number is far less,
and the imperfection of the assessment of it is perhaps not such a serious deficiency
at the present stage of the inquiry
as might have been anticipated. An
approximate analysis of a polar metaphase
in a squash preparation is contained in
Fig. 250. Multivalents, if present, cannot
easily be recognized or allowed for, but
ignoring their possible existence, the
analysis records approximately 113 pairs
and 37 univalents. The sporophytic
chromosome number of L. Selago cannot
therefore be less than 260.
The interpretation of these facts is not
at once obvious. In the British species of
Lycopodium, with the exception of L. anno-
tinum, which almost certainly links up
with L. clavatum, the cytological evidence
as a whole can only underline their com-
plete dissimilarity from one another, and
one must recognize in them the represen-
tatives of phyletic lines which have been
so long separated that their cytological
connexion, if it ever existed, has become
completely obscured. They seem now to
be far more different from each other than
are the genera or even groups of genera
of the Polypodiaceous ferns. This is per-
haps a sign of antiquity. Yet suddenly,
in L. Selago, we find a species which is
behaving like a hybrid that has succeeded in covering up a defective meiotic process
by a highly successful reproduction by means of bulbils. Can this really be the
case? To investigate it further we need to assemble observations on meiosis not from
Britain only but from all over the world. It may be that the British plants are peculiar,*
or it may be that under certain conditions failure of pairing may be induced from meta-
bolic causes and not from lack of homology among the chromosomes. In that case the
undoubted gametophytes which have been found might result from local strains or under
metabolic conditions in which pairing was not irregular. Before the idea of hybridity
can be accepted some explanation for the sexual prothalli must in any case be found. It
would be unwise, at this stage, to prejudge the issue as to what this explanation might be,
but if the signs of hybridity are borne out by further study, one may be quite certain,
first, that the parent species, wherever they may once have been, are unlikely now to be
* As this chapter goes to press mention can be made of one observation on a non-British plant. At
Storlien in Sweden unpaired chromosomes were seen again in the summer of 1948.
Fig. 251. Silhouette of Lycopodium annotinum L. from
Switzerland. Natural size. This specimen pro-
vided a root-tip count.
252
Fig. 252. Isoetes acustris L., two growth forms from Lake Windermere,
from pressed specimens. Natural size.
253
THE LYCOPODS (CLUBMOSSES)
in Britain, and secondly, that L. Selago, if it is a hybrid, is Hkely to be the most ancient
impure species that cytology has so far detected.
Leaving Lycopodium we come to the two heterosporous * genera Isoetes and Selaginella.
The aquatic Quillworts, Isoetes lacustris L. (Fig. 252) and /. echinospora Durieu, are not
unfamiliar inhabitants of the pure waters of our glacial lakes and mountain tarns, and
their conditions of culture are fortunately of the simplest. It is only necessary to put a
httle garden soil at the bottom of an inverted bell-jar, which is then filled up with water,
/soeAes n= 54
Fig. 253. Explanatory diagram to Fig. 256a. x 2000.
and Isoetes will grow indefinitely in a laboratory if kept near a cool north window. In
addition to the two species recognized taxonomically, there are undoubtedly many true-
breeding strains characteristic of different lakes, which form local inbreeding com-
munities. Maximum stature appears to be partly under genetical control and ranges
from the fairly small plants with leaves 4 in. or so long, characteristic of Windermere, to
immense plants with leaves over a foot in length such as the var. Morei Moore found
in Loch Bray in County Wicklow in Ireland. These size diflferences are constantly
maintained in culture and do not appear to be environmentally induced growth forms.
* By heterosporous is meant the production of spores of two sizes, the large, or megaspores, being few
in number and heavily stored with food for the production of exclusively female prothalli; the small, or
microspores, being formed in greater numbers destined to produce only diminutive male prothalli. The
microspores oi Isoetes and Selaginella are the equivalent of the pollen grains in the higher plants.
THE LYCOPODS (CLUBMOSSES)
A viviparous form in which vegetative buds replace sporangia is also known from Win-
dermere and no doubt from other places.
Material kept in culture for many years and used for cytological study of root tips
has been as follows :
/. echinospora, normal form from County Kerry in Ireland.
/. lacustris, normal form from Windermere (Fig. 252) ; viviparous form from Winder-
mere; var. Morei from County Wicklow, Ireland, with very long leaves; form with
leaves intermediate in length, Wales.
All these plants proved to be cytologically indistinguishable with more than 100
chromosomes in their roots. Examination of meiosis was therefore confined to material
from the most easily accessible wild locality, namely, the /. lacustris colonies in Winder-
mere.
As in many, or perhaps all, heterosporous Pteridophyta the maturation of mega-
sporangia tends to precede that of microsporangia. In /, lacustris the former takes
place in July, but the latter, which is greatly to be preferred for cytological purposes,
only at the end of August. Figs. 253 and 256 a show a metaphase plate obtained at this
season from Windermere, and the peculiar shapes of the chromosomes already seen in
Lycopodium and Equisetum are again displayed. For this reason, as in the other cases,
accurate counting is difficult. The cell in question contains not less than 54 pairs, nor
more than 56, and this, unfortunately, is as near to accuracy as has so far been attained.
The other British species, Isoetes hystrix Durieu, is both so different and so unfamiliar
that a word of description about it may not be out of place. This minute terrestrial
species has its headquarters geographically in the Mediterranean regions, where it
is sparingly met with in isolated small colonies, spread from the south of France to
North Africa, occupying sites of an extremely xerothermal character. It is confined
to areas which are regularly moistened with flood water in winter, during which season
it vegetates freely and may become totally submerged. In summer, on the other hand,
the sites dry out completely, and all the plants inhabiting them pass the hot season in a
dormant and desiccated condition, either as subterranean geophytes {Isoetes) or as seeds
{Juncus capitatus). The 'Isoetetalia', as these localities are called in ecological language,
have been studied several times on the Continent by Braun-Blanquet and his school,
though I am not aware of any similar study of the only two existing British sites, which
are in Guernsey and on the Lizard respectively. That Isoetes hystrix is present in Guern-
sey has been known since i860. The Lizard site, though detected a century ago, was so
difficult to find again that the record was disbelieved until 1933, when plants o{ Isoetes
were re-found there in some quantity by R. Melville of Kew whilst searching for Juncus
capitatus. I am indebted to Dr Melville for instructions as to how to reach the Lizard
site, and I have also been able to collect material from Guernsey, on both occasions the
season being early April, at which time the vegetative season is almost at an end and
meiosis of the last microspores in progress. By June all traces of the leaves have vanished
from above ground, but, curiously enough, in the skilful hands of Mr Ashby, Isoetes
hystrix from both localities proved surprisingly easy to cultivate. Intact sods containing
it, together with other geophytes such as Scilla verna, were lifted in 1938 and 1939
and transferred to small pots in the roof greenhouse of Manchester University where
255
THE LYCOPODS (CLUBMOSSES)
they have been maintained ever since. In order to respect their evident need for a dry-
dormant period, they are dried out annually and placed in a cupboard for some months,
the season chosen being the winter instead of the summer in order to give them the
benefit of such sunshine as is available in Manchester. Under this treatment they have
n
t
Fig. 254. Isoetes hystrix Durieu. a. Pot of plants from the Lizard, Cornwall, growing in cultivation. Half
natural size. b. Two wild specimens pickled as found, the left-hand plant from the Lizard, the
right-hand plant from Guernsey. (Note the double crown in the latter case.) Natural size.
increased very greatly in size, as the photograph of Fig. 254^ taken in 1945 will show.
This is half natural size, and yet the plants in it resemble the full stature of the original
wild specimens which are shown, pickled at the time of collection, in Fig. 254^. In this,
the left-hand specimen is a plant from the Lizard, as are those of Fig. 254a; the right-
256
I
THE LYCOPODS (CLUBMOSSES)
hand specimen of Fig. 254 /> is from Guernsey and was especially selected as an
abnormally large plant with a branched stock.
The remarkably disjunct character o^ Isoetes hystrix localities makes it certain that in
each must be a little inbreeding community, isolated for long periods of time, and it is
Fig. 255. Isoetes hystrix Durieu growing in cultivation to show characteristic differences of habit in
material from different sources. The tall left-hand plant came from Morocco, the right-hand plant
from the Lizard. About a quarter natural size.
Fig. 256. Chromosomes of /joe/ej. All x 1000. a. /. /ac?/^<m L. first meiotic metaphase, in permanent
acetocarmine. x 1000. For explanatory diagram see Fig. 253. b. I. hystrix Durieu from the
Lizard, diakinesis in permanent acetocarmine. x 1000. n= 10. c. The same, showing metaphase
in a section, x 1000. d. Root of/, hystrix from Morocco in a section, stained with Feulgen's
method, to show chromosome size, x 1000. 2n=:20.
therefore to be expected that local populations should show genetically based differences
of the same type as those already met with in the aquatic species, and this is undoubtedly
the case. A detailed comparison between the populations of Guernsey and of the Lizard
has not been made owing to the inaccessibility of Guernsey during the war years. At the
time of my visit to that island in 1939, only a token collection of live plants had been
MFC
257
17
THE LYCOPODS (CLUBMOSSES)
brought back, which was shortly reduced to a single plant by inadvertence, thereby pro-
viding too scanty a basis for comparison. Fig. 255, on the other hand, shows an old pot
plant from the Lizard (right) growing beside anold pot plant from Morocco (left) provided
I
Fig. 257. Prothallus of Isoetes hystrix Durieu from a glycerine jelly mount, a. Germinated megaspore
showing rhizoids and detached spore coat (inset), x 50. b. Rhizoids enlarged x 500.
some years before by Professor T. G. B. Osborn. Two specimens from Morocco were kept
alive in Manchester for many years by means of the cultural treatment described above,
and both differed strikingly and constantly from the British plants both in stature and
258
THE LYCOPODS (CLUBMOSSES)
mode of growth of the leaves. In British material the leaves grow spirally and are
pressed down to the ground. In the African specimens they are stiffly erect and con-
siderably longer. Whether this means that /. hystrix should be split into more species
than one is, however, for taxonomists to say.
Fig. 2^6 b-d gives the cytological facts for the African and British material. Fig. 256c
shows microspore mother cells in /. hystrix from the Lizard, fixed in the field and subse-
quently sectioned and stained in haematoxylin. Fig. 2^6b shows a squash preparation
of the same material at the same magnification: n= 10. The diminutive chromosomes
and their low number make a very striking contrast with everything which we have
hitherto seen, and the comparison of chromosome size can equally be made from the
root of the Moroccan plant (Fig. 256^), in which approximately 20 somatic chromo-
somes are visible, at the standard magnification (1000 diameters), used almost through-
out this book.
Before leaving Isoetes it may be of interest to add a note about the prothalli of/, hystrix,
since, as far as I am aware, these have never before been seen. They were detected
by Mr Ashby in the material from Morocco, and I am indebted to him for the
photograph in Fig. 257. A germinated megaspore from which the spore coat has become
detached is shown in Fig. 257 a, and the great length of the rhizoids is a striking
characteristic (see also Fig. 257^). Since rhizoids are absent from the aquatic species and
are as a rule poorly developed in the otherwise not dissimilar prothalli o^ Selaginella, their
presence here in association with the rather extreme habitat conditions of the species may
perhaps be a point of ecological importance.
The second heterosporous genus, Selaginella
(Fig. 258), need not detain us long, since we
have only one species in Britain, S. spinulosa
A.Br., relatively common and vigorously
fertile in all our mountain districts. The only
cytological difficulty it presents is due to its
small size, but roots can be fixed at any
time and meiosis can be obtained in July. Fig. 258. Selaginella helvetica (L.) Link.
The chromosomes are, however, minute. Silhouette of a dried specimen of the plant
,_^, . 1 /T-- \ 1 used, from Switzerland. Natural size.
Iheir number (rig. 259a) appears to be 9
at meiosis and 18 (Fig. 260c) in roots. These numbers were found again in the other
two European species. S. helvetica (L.) Link was fixed in the field in Switzerland in July
1938, and root tips gave the result shown in Figs. 259^ and 260^. S. denticulata (L.) Link
was collected in North Italy in the same year by Professor Lang, who brought it ahve to
Manchester, where it coned in cultivation. Fig. 259 c^ shows the side view of metaphase
in a megaspore mother cell in which the diminutive size of the group of chromosomes
is so extreme that they are scarcely visible. Their number, however, appears to be 9
in this cell and 18 as before in roots.
This low chromosome number, comparing closely only with Isoetes hystrix among all the
living Pteridophyta so far studied, is a matter both of surprise and of importance. The
genus Selaginella is very large, with over 800 species, most of which are tropical. The three
European species, however, belong to widely different sections of the genus, S. spinulosa
^^^^^^^4^
259
17-2
4
Fig. 259. Chromosomes o[ Selaginella. a. S. spimdosa A.Br, diakinesis in fresh acetocarmine. x 1000.
For explanatory diagram see Fig. 260a. n = 9. b. The same, metaphase of the second meiotic
division, permanent acetocarmine. x 1500. For explanatory diagram see Fig. 2606. c. The same,
mitosis in a root, x 1500. For explanatory diagram see Fig. 260c. 2«= 18. d. S. denticulata
(L.) Link. Part of a section through a megasporangium with the single spore mother cell which
it contains at metaphase of the first meiotic division. The diminutive plate of tiny chromosomes
lies in the plane of the arrow, x 1000. e. S. helvetica (L.) Link. Mitosis in a root, x 1500. For
explanatory diagram see Fig. 260^. x 1500. 2n= 18.
0
S.spinulosa ^n=/8
d
S.hdveticz ^n=l8
Selaginella spinulosa n=9
Fig. 260. Explanatory diagram to Fig. 259 a, b, c, e. All x 2000.
260
THE LYCOPODS (CLUBMOSSES)
being radially symmetrical, with uniform leaves, and no rhizophores, the other two
species being of the dorsiventral type with rhizophores. Their very close cytological
similarity must mean that considerable uniformity is likely to prevail throughout the
genus, and yet in spite of this, or even perhaps because of the low chromosome number,
far more active speciation seems to have been at work here than in most other living
members of the group. Why such a low chromosome number should have persisted here
and not elsewhere in the Pteridophyta is a question which we are not likely to answer,
but the fact that it has done so is of importance to this inquiry as a clear indication that
the high chromosome numbers found elsewhere and increasingly so in the most ancient
groups are, in a sense, secondary. The Pteridophyta seem almost certainly to have pos-
sessed in the past, and to have retained in these two genera, the simple nuclear state that
the modern Flowering Plants still for the most part display. In this one particular of
chromosome number the heterosporous members of the Lycopodiales seem to have re-
tained a primitive condition, though little else about them would suggest that they are
simple.
SUMMARY
In a review of all the British species of Lycopodium and Isoetes and of all the European
species oi Selaginella special morphological interest attaches perhaps to the new observa-
tions on the prothalli and growth forms of the terrestrial species of Isoetes [I. hystrix) .
Special points of cytological interest are the wide diversity and peculiar shapes of the
chromosomes of the species of Lycopodium together with the very remarkable display of
unpaired chromosomes suggesting hybridity in L. Selago. Selaginella and Isoetes hystrix
are exceptional among all known members of the Pteridophyta for the small size and low
number of their chromosomes. The other cytological facts provided in the chapter are
contained in the following list:
Species
Source
2«
n
Isoetes hystrix Durieu
Cornwall
20
10
Guernsey
20
•
Morocco
c. 20
•
/. lacustris L.
Windermere
Not less than 100
54-5C
/. echinospora Durieu
West Ireland
Not less than 100
(probably identical
with preceding)
•
Selaginella spinulosa A.Br.
Lake District
18
9
S. helvetica (L.) Link
Switzerland
18
,
S. denliculata (L.) Link
Italy
18
9
Lycopodium inundatum L.
Scotland
•
78
L. clavatum L.
Lake District
68
34
L. annotinum L.
>>
c. 68
Sweden
,
34
L. alpinum L.
Wales
Probably 48
24-5
Lake District
,
,
L. Selago L.
jj
'260'
(see text)
Irregul
Scotland
•
ji
Wales
,
«4
Sweden
.
»)
261
CHAPTER 16
THE ANCIENT FERNS
We have almost reached the end of this inquiry, and now it only remains to supplement
the account of the modern ferns given earUer in the book by adding some facts for the
ancient ones, choosing, as before, those groups with British representatives. In this case
there are three. Of the Eusporangiatae, thought to be the most ancient of all the
living ferns by Bower, we have the Ophioglossaceae, represented by the Adder's Tongue
{Ophioglossum) and the Moonwort {Botrychium). Of the Osmundaceae, placed by Bower
on the border between the Eusporangiatae and the more modern Leptosporangiatae, we
have Osmunda, and as an example of a Leptosporangiate group more primitive than the
Polypodiaceae we have the Hymenophyllaceae or Filmy Ferns, of which there are three
British representatives distributed among the two * genera which it contains, namely,
Trichomanes and Hymenophyllum. Taking these groups in the order mentioned we may
start with the Ophioglossaceae.
Bower's reasons for regarding the Eusporangiatae, represented in Britain by Ophio-
glossum and Botrychium, as the most ancient of all living ferns are purely morphological
and depend on the fact that in the relatively massive construction of all parts of the
plant, especially of the sporangia and sex organs, these genera contrast strongly with
the undoubtedly modern Leptosporangiate ferns in features which Bower interprets as
primitive. Direct fossil record of these genera is wholly lacking, but granted the correct-
ness of the reasoning, which there seems no reason to doubt, it is certainly easier to derive
the living genera in imagination from certain extinct groups of the Carboniferous Period
known as the Coenopterideae than from any living ferns.
The Eusporangiatae in the world as a whole are commonly subdivided into two main
groups, the Ophioglossaceae and the Marattiaceae, the latter consisting of a few genera
of archaic tree ferns, all of which are tropical, and for this reason excluded from this
survey at its present stage. The Ophioglossaceae contain only three genera, Ophio-
glossum, Botrychium and the tropical Helminthostachys, and therefore only the last will be
excluded.
The British species of Ophioglossum are two, 0. vulgatum L., the Common Adder's
Tongue, widely spread over the whole country in moist pastures and probably more
abundant, because of its inconspicuousness, than is generally supposed, and 0. lusitani-
cum L., a much smaller Mediterranean species with a well-known British locahty on the
island of Guernsey. Material of both these species has been available to me, and for con-
venience 0. lusitanicum will be taken first.
The appearance of this httle plant at the end of its vegetative season (late April in
Guernsey) may be seen in Fig. 2610. It is a tiny fern forming a dense, close sward in
which some characteristic litde bulbous Monocotyledons, notably Scilla verna and
* It should perhaps be noted here that Copeland (1947) replaces this simple classification of the
Hymenophyllaceae by no less than thirty genera, most of which are confined to the southern hemisphere.
262
THE ANCIENT FERNS
Romulea columnae, are also present. On the island of Guernsey, from which my material
comes, Ophioglossum lusitanicum is a markedly xerothermal plant, in some ways resembling
in its requirements Isoetes hystrix. It is found in a few shallow hollows near the summit of
cliffs on the south coast of the island, which enjoy very full
insolation and almost complete protection from north, west
and east. It is not surprising, therefore, that in cultivation it
seems to appreciate an annual period of desiccation as does
/. hystrix, and, in fact, it has been given treatment exactly
corresponding to that described on p. 256 since 1939, when
it was first collected, and no plants have been lost by death
since that date. Owing to neglect during the war, however,
and to an accidental shortening of the growing season for
several successive years, sporangia have ceased to be pro-
duced. For this reason the cytological study has been less
complete than would otherwise have been possible.
Sections of roots show at once what has been encountered
so often in the Pteridophyta, namely, that
the chromosomes are so numerous that
mitotic figures are virtually useless for
evaluating them. Fig. 262 c, however,
shows meiosis with an explanatory dia-
gram in Fig. 263 a. It was obtained in
1940 from the first crop of fronds to be
produced in cultivation. Though good
enough for an approximate count, the
preparation is unfortunately not quite ad-
equate to establish the gametic number
with final accuracy. The approximate
value is, however, unquestionably of the
order of w=i28, though the number of
cells available is too small to exclude a
possible margin of error of one or two
chromosomes. Since the difference be-
tween 128 and, say, 126 is of importance
in interpreting the whole position, as
anyone who has followed the description
of Cystopteris will understand, it is desir-
able not to prejudice the argument at this
stage by claiming a premature finality. The haploid number for Ophioglossum lusitanicum
must therefore for the moment remain in the uncertain position of^n = c. 128 or, more
precisely, 'not less than 125 nor more than 130'. Even so, the reader may feel that here
are quite enough chromosomes for such a tiny plant.
0. vulgatum, the Common Adder's Tongue, is not infrequent in moist meadows near
Manchester, and I was fortunate in having been able to collect fairly abundant material
263
Fig. 261. a. Ophioglossum lusitanicum L. from a dried
specimen from the Guernsey locality. Natural
size. b. Ophioglossum vulgatum L. from York-
shire, from a dried specimen. Natural size.
THE ANCIENT FERNS
of both generations from this region. Since the prothalh have not, so far as I am
aware, been described previously from England, though they have doubtless been
found by other workers, it may perhaps be of interest to give some biological notes
about them.
Prothalh of 0. vulgatum (Fig. 264) were found by me on many occasions in the summer
of 1 94 1 between the months of May and September in a field near Adlington in Che-
shire, which had been long under grass and from which turf had been cut a few weeks
before the search began. Attention had been attracted to the field by the circumstance
that young Ophioglossum fronds with severed petioles were found in the cut turf which had
been laid as a lawn in a neighbouring garden. When the site of the cutting was visited
S
. «t. ^
r
• »
Fig. 262. Meiosis in British Ophioglossum in balsam after acetocarmine. x 1000. For explanatory
diagrams see Fig. 263. a. O. liisitanicum L. b. 0. vulgatum L.
some fully expanded fertile fronds of Ophioglossum were found to be growing in the un-
disturbed grass at the edge of the bared patch. Study of the rest of the field showed that
local small colonies of the fern were scattered about in various parts, too far away from
each other to be easily the result of passive vegetative expansion by means of the root
buds, and the possibility of inoculation having occurred by germinated spores seemed
favourable. To search for these, sods of the soil from which the turf had been removed
were dug out from the immediate neighbourhood of the untouched adult plants and
carefully crumbled to pieces over a sheet of white paper. As it turned out, prothalh
were detected surprisingly easily, in most cases at a depth corresponding to about 5 in.
below what must have been the original surface of the grass before the turf had been
removed.
The prothalh of Ophioglossum are little, contorted, wormlike objects, very like those of
Psilotum referred to in Chapter 14, though without any trace of vascular tissue. A group
264
THE ANCIENT FERNS
of larger and smaller ones is shown, natural size, in Fig. 264, and the larger ones can be
seen to be branched. Some sections through fertile regions are shown in Fig. 266, and the
*^A ^
0. lusifanicum n- c. I 28
Ophio^lossum \/ulgdtum n = c. dS6
Fig. 263. Explanatory diagrams to Fig. 262. x 1500.
soft undifferentiated central tissue can easily be seen. As in Psilotum, the prothallus lives
saprophytically with the aid of an endophytic fungus, and in most details my own obser-
vations merely confirm the very excellent general description given by Bruchmann in
265
THE ANCIENT FERNS
1904. The sunken sex organs are well seen in Fig. 266, with some additional details of the
opening of archegonia, etc., depicted in Fig. 265. As was observed by Bruchmann, June
is the first month in which mature sex organs can be found, but from June till September
it is easy to observe both the liberation of spermatozoids and the opening and impregna-
V
c
<
K
Fig. 264. Prothalli of Ophioglossum vulgatum L. found in Cheshire
and preserved in alcohol. Natural size.
Fig. 265. Sexual reproduction in living prothalli of Ophioglossum vulgatum L. in the month of July.
c. Part of a prothallus in water showing two of the neck cells of a freshly opened archegonium
opposite the arrow, x about 100. b. A spermatozoid recently emerged and killed in iodine to
show the cilia, x about 200.
tion of archegonia. Some observations to illustrate this are contained in Fig. 265, in
which Fig. 265/) shows a newly emerged male gamete killed with a drop of iodine
and photographed at once, and Fig. 265 a a living prothallus with the neck of a newly
opened archegonium projecting beyond the solid tissue. The size of the neck cells in
relation to the whole prothallus can be judged by noticing the white patch on the lower
side of the picture which marks the other side of the prothallus. A short time after
266
/^
^J ^^
d
»
-' ^ •
.\-7 • • ' -
/ ♦ - •
• »V:v
aV ^
•V •' ,
A-' '
..»••• \
A/ >
. .
( • •; -
- y<
y- ^
Vi .•
- ♦• *
■ , • ' - '
»■ »
.'.
^^•?-
4
♦'^ . ' >^^
""
< ■ o
7.,
I
Fig. 266. Some anatomical details of the prothalli oWphioglossum vulgatum L., from sections, a. A recently
impregnated archegonium with two spermatozoids in contact with the egg (somewhat shrunk on
fixing). X200. b. Immature unopened archegonium. x 400. c. Part of a transverse section
with two antheridia, the upper one empty, the lower containing young spermatozoids. The
dehiscence cell appears dark in both, x 200. d. The same with spermatozoids emerging, x 200.
e. A complete transverse section through the fertile region off, showing various stages of antheridia.
x 100. /. The same, showing the archegonium of Fig. 265a (pointing downwards), x 100.
267
THE ANCIENT FERNS
opening, the neck cells of the topmost tier generally seem to loosen from each other and
are thrown off, after which the neck closes whether fertilization has occurred or not. An
early stage in the process of fertilization is illustrated in Fig. 266a, but no developing
embryos were seen. The signs of nuclear fusion in the first few days after insemination
were, however, sufficiently clear to leave little doubt that the gametes are fully func-
tional.*
This fact is of some importance, for otherwise the question might have been raised as
to whether the nuclear condition subsequently found in the sporophyte is compatible with
effective sexuality. The reader will by now have become somewhat acclimatized to a
record of high chromosome numbers, especially after seeing Equisetum and Tmesipteris
(Chapters 13 and 14). In Ophioglossum vulgatum, however, we have an even more ex-
treme case. As Figs. 262 b and 263 b will show, the chromosomes are sm^aller than those
of 0. lusitanicum and about twice as numerous. There is no sign of multivalents, so that
we need not ask whether this is merely a polyploid strain. The number is difficult to
assess with final accuracy, but allowing the maximum margin of uncertainty that the
observations require, it may be said with confidence that in 0. vulgatum the gametic
chromosome number is not less than 250 nor more than 260, and that the correct figure
lies somewhere between these limits. This is the highest chromosome number yet dis-
covered in a wild species in the plant kingdom, for it means that there are more than 500
chromosomes per cell in the sporophytic tissue.
It is so probable from these approximate figures that 0. vulgatum has exactly twice the
chromosome number of 0. lusitanicum, that this may be safely assumed without further
demonstration, and it is here that the range of possible numbers already mentioned for
the latter species becomes of importance. We are clearly dealing with the upper mem-
bers of a polyploid series, the lower members of which are unknown, and we cannot at
the moment diagnose the fundamental chromosome number until either the lower mem-
bers are found or greater accuracy can be introduced into our study of the higher ones.
If, for example, the correct figures for the two British species could be shown to be
exactly 128 and 256 respectively, we should be unquestionably dealing with a polyploid
* In an attempt to observe the cytological details of fertilization a number of small prothalli were
inseminated on 21 June 1940, and fixed at intervals thereafter. The presence of opened archegonia was
checked in each case by microscopic examination at the time of insemination. The first specimen, which
was fixed within an hour of the start of the experiment, namely, at 7 p.m. on 21 June, contained the
archegonium with the still open neck canal photographed in Fig. 266a. Three hours later a second speci-
men showed an archegonium with a male nucleus somewhat swollen but definitely inside the egg. On
the following day at i p.m. a third specimen was fixed, which on sectioning showed a female nucleus with
a crescentic reticulated mass spread partly over its surface, which resembled very closely a fertilization
stage seen in Equisetum sylvaticum, so that it can safely be interpreted in the same way. Unfortunately, this
specimen had been cut by the microtome knife rather inconveniently, the centre of the crescent being in
one section and the two arms in the next, so that a convincing photograph is difficult to obtain. A fourth
specimen fixed at the end of a week showed no trace of unabsorbed male nuclei but also no trace of
division of the egg. The only difference between this and the behaviour oi Equisetum or Dryopteris, in both
of which I have successfully followed the details of fertilization, is the much greater slowness of Ophio-
glossum. The late fusion stage described after 18 hours is reached in three in Equisetum, and in Dryopteris
the first division of the egg occurs after a week. It is therefore probable that had observations been con-
tinued for longer the cleavage stages might have been seen. As it is, there is only proof of nuclear fusion.
268
THE ANCIENT FERNS
series on the number 8, very familiar in the flowering plants though not so far en-
countered in the ferns ; in that case we might either
hope to find, or could postulate the former exis-
tence of, species with the intermediate numbers 8,
1 6, 32, 64. If, on the other hand, the correct
numbers for the British species are 126 and 252
respectively, we have an extension of the position
already encountered in Cystopteris, in which the
immediate monoploid number was 42, though this
might in turn be referable back to a basic haploid
of 7. Since present accuracy does not enable us
to distinguish between these two possibilities, we
must leave the matter sub judice at this point. It is
evident, however, that the two British species of
Ophioglossum have opened up some very interesting
problems in a very interesting genus which could
profitably be further pursued in other parts of the
world.
In comparison with Ophioglossum, the one British
species of Botrychium (Fig. 267) offers little of
special interest from a cytological point of view.
From the technician's standpoint it is more
difficult to handle than Ophioglossum, in that its
seasonal periodicity is less convenient. In both
species of Ophioglossum meiosis takes place when
the fertile fronds are well above the ground and
the sporangia easily seen and tested. In 0. vul-
gatum itself the fertile spike is then about 2 cm.
long, and the month in the north of England is
June. The Moonwort, Botrychium lunaria (L.) Sw.,
matures its spores much earlier when the fronds
are in the act of piercing the ground and whilst the
sporangia are still closely enveloped by the unex-
panded sterile portion. This means that they are
very difficult indeed to find in the wild until
they are too old, unless very exact knowledge of
localities has been assembled in a previous year.
On the other hand, cultivation presents difficulties,
but under suitable conditions sods containing
plants can be maintained alive through at least
one season, and this, in my experience, is the less
difficult way of obtaining fixations.
Figs. 268 and 269 show meiosis in material of
B. lunaria obtained from Teesdale in summer but
Fig. 267. Botrychium lunaria (L.) Sw.
from Teesdale. Natural size.
269
THE ANCIENT FERNS
fixed in cultivation in April of the following year. The chromosomes are fairly large
and very distinct (Figs. 268-269). Their number is ^ = 45. This number has not so far
been found elsewhere in the ferns, a fact which is of no special importance. Further
comment must, however, await the study of a greater number of species.
Turning now to the Hymenophyllaceae we have three British species to consider.
The Killarney Fern, our only species of Trichomanes, is almost confined to the very moist
conditions of the west of Ireland from whence it derives its name. The two species of
Hymenophyllum, the Filmy Ferns of our mountains and of a few lowland localities such as
the Common at Tunbridge Wells from which H. tunbridgense has long since disappeared,
♦r * * + *
♦ -* ♦ . ^
% *
f
fi
^i
Fig. 268. Meiosis in Botrychium lunaria
(L.) Sw., permanent acetocarmine.
X 1000. For explanatory diagram
see Fig. 269. ^ = 45.
Bof-rychium n = 45
Fig. 269. Explanatory diagram to
Fig. 268. X 1500.
are widespread and not difficult to find, though H. unilaterale is the more abundant of the
two in most parts of England. Both rank among our smallest vascular plants though
they may each cover extensive surfaces with a dense mat of saturated, translucent
foliage on suitable shaded rocks or even, at the extreme west of our islands, on the bare
ground.
Considering the very delicate nature of their leaves all British members of the
Hymenophyllaceae are surprisingly hardy in cultivation though they are not always fer-
tile under these conditions, and fixations should therefore, if possible, be made in their
native haunts. I was unable to do this with Trichomanes, but I was fortunate in receiving
some very fertile leaves of Irish origin from the Botanic Garden at Glasnevin in Dublin,
one of which is represented in Fig. 270. The chromosomes of this species appear in
Figs. 271 and 272. They are fairly large and their number is n = 72. The longevity of
this leaf was so astonishing, it being remembered that the lamina is only two cells thick,
that the facts are worthy of mention. It remained shut up in a tin box for four weeks in
total darkness after the fixings had been taken, at the end of which time it was still fresh
and green and the sori had increased considerably in size by comparison with the other
specimen which had been pressed at once.
270
Fig. 270. Trichomanes radicans Sw., from a pressed specimen of the leaf used, supplied by Dublin Botanic
Garden and probably of native Irish origin. Natural size.
271
THE ANCIENT FERNS
With regard to Hymenophyllum, H. unilaterale Bory was fixed in the field in Borrowdale
in the Lake District in early July. Its chromosomes are shown in Fig. 274(2 and they are
very surprising. In spite of the minute size of the plant they are the largest individual
1^
Fig. 271. Meiosis in Trichomanes radicans Sw., permanent acetocarmine. x 1000.
For explanatory diagram see Fig. 272. n—']2.
Trichomanes " n -72
Fig. 272. Explanatory diagram to Fig. 271. x 1500.
chromosomes yet recorded in the Pteridophyta and their number is only n= 18. The
same kind of surprise can be felt in viewing H. tunbridgense (L.) Sm. (Fig. 274^). This
was fixed in the moss house of Glasgow Botanic Garden which seems to suit it unusually
272
THE ANCIENT FERNS
well, for, in a rather dry July, it was fruiting far more luxuriantly there than in the local
wild habitat near the shore of Loch Lomond. The chromosome number is even lower
than that of the other species and there is a greater range of dimensions. As Fig. 274^
will show, H. tunbridgense has « = 1 3, with some individual chromosomes as large as
those of//, unilaterale but others quite small.
The significance of these two numbers and of the
range of chromosome sizes in the last species cannot be
known unless other species can also be studied. It is
obvious that the two species themselves are quite
distinct, in spite of a superficial resemblance caused by
a general community of habit and habitat which tends
to obscure their morphological differences at a first
acquaintance. Which of the two chromosome numbers
should be thought of as the more primitive cannot
yet be known, though the relation between n = 18
(//. unilaterale) and n = 72 ( Trichomanes radicans) may
suggest that the former number at least is rather deeply
seated in the group. It is therefore possible that the
present state of Hymenophyllum tunbridgense may be the
more derived condition, in which case it would be the
only instance which has so far come before us in which
we can imagine that evolution might have entailed a
reduction of chromosome number.
Be that as it may, it is clear that we have here a quite
unusually interesting group for further study, though
such study should preferably be carried out in the
southern hemisphere where the majority of living species are to be found. That they
would make wonderful cytological material for other purposes, if only their habitat
requirements were less inconvenient, is perhaps indicated by the demonstration of spiral
structure in H. tunbridgense at anaphase of the first meiotic division, which is appended
in Fig. 275. Recollection of the spiral may perhaps diminish the surprise at first caused
by the large size of the chromosomes, though a problem of great interest is, nevertheless,
present. Bulk for bulk it looks as though a single chromosome of H. tunbridgense is
several times larger than the whole nuclear apparatus of, say, Selaginella, and yet who
will say that the one is more effective than the other? The extreme contrast between
Hymenophyllum, Ophioglossum and Selaginella may indeed impress forcibly upon us how
superficial are the comparisons which we have been making. Selaginella alone should
warn us how minute is the quantity of genetical material actually sufficient for the
development of an organism, and the enormous numbers of chromosomes in, say,
Ophioglossum, no less than the massive size of the chromosomes in Hymenophyllum must
be only of secondary importance to them, interesting as these characters may be as a
guide to developmental changes in the past.
Restraining ourselves, however, from premature discussion of philosophic questions
we may pass on to our last and in some ways most important group, the Osmundaceae.
18
Fig. 273. Hymenophyllum tunbridgense
(L.) Sm., silhouette of a living
frond of the strain used, grown in
cuhivation but obtained from Loch
Lomond, Scotland. Natural size.
MFC
273
THE ANCIENT FERNS
Osmunda regalis, the only European member of it, has already been described in some
detail in Chapter 3. A single species, however, is insufficient basis for discussion of a
group, and in this one instance it seems desirable to break our general rule of avoidance
of botanic gardens in order to supplement it. This is perhaps less harmful here than it
V
•4
s ^ \%
— , If
>•''#
V*
Fig. 274. Meiosis in British Hymenophyllum, permanent acetocarmine. x 1000.
a. H. milaterale Bory. n= 18. b. H. tunbridgense (L.) Sm. n= 13.
Fig. 275. Hymenophyllum tunbridgense (L.) Sm. Spiral structure of chromosomes
at anaphase of the first meiotic division, x 2000.
might be elsewhere, because all members of the group are so distinctive and possess such
well-defined geographical ranges that confusion between them is virtually impossible,
even in the absence of all records of the actual sources of collections.
The living Osmundaceae consist of the three genera, Osmunda, Todea and Leptopteris.
The first contains about ten mainly temperate species, collectively distributed over a
very large part of the earth's surface. The second, Todea, is monotypic, consisting only
of T. Barbara (L.) Moore (sometimes known as T. africana Willd.), a subtropical plant
Fig. 276 (opposite). Osmunda palustris. Silhouette of living leaves from a horticultural source, a, sterile;
b, fertile. Natural size.
THE ANCIENT FERNS
of South Africa and Australia. Leptopteris, which resembles Todea in many ways but
differs from it by the fact that the leaf texture is 'filmy' as in the Hymenophyllaceae, is
confined to New Guinea, Polynesia and New Zealand and contains about seven species.
This very modest array of living forms is in striking contrast to the wealth of the fossil
record. The most important information regarding this is still to be found in the classic
researches of Kidston and Gwynne-Vaughan (1907-14), who described the structure of
all anatomically preserved specimens known at that time, and very few have been added
since. Stem and petiole structures, so like those of the living Osmunda that they could be
thought of as cogeneric with it, have been found in many countries and at many geo-
logical levels, as the following list of'Osmundites' will show:
Table of species q/" Osmundites with anatomically preserved structure known to Kidston and
Gwynne-Vaughan^ 1907-14
Horizon Species
Tertiary of Paraguay Osmundites Carnieri
Tertiary of Spitzbergen 0. Spetsbergensis
Miocene of Hungary 0. Schemnitzensis
Eocene of Britain 0. Dowkeri
Lower Cretaceous of British Columbia 0. Skidegatensis
Upper Jurassic of South Africa 0. Kolbei
Jurassic of New Zealand 0. Dunlopi
,, „ O. Gibbiana
At a still earlier level, other stems and petioles have been found, so like those of
Osmunda and Osmundites that they must certainly be referred to the same group though
not to the same genus, since they differ from all the living and late fossil genera not only
in being larger but also in being in certain respects more complex. In the Permian of
Russia alone no less than four distinct genera have been described {^alesskya, Thamnopteris,
Anemorrhoea and Bathypteris) . Further back still the resemblance of Osmundaceous
sporangia to certain unattached fruit bodies of Carboniferous age may perhaps indicate
an ancestral type in the Coal Measures.
A direct record of this kind is something unexampled elsewhere in the ferns, and it is,
indeed, only to be compared with Equisetum among living vascular plants. This is the
reason for the special importance of the group in the present context, for, perhaps even
more than in Equisetum, we know that it, and perhaps even certain species in it, have
existed unchanged for very long periods of time, although its most important evolutionary
outburst was at the beginning.
All the species available in cultivation in the botanic gardens of Europe have been
examined, and, in addition, I have received one specimen of Osmunda javanica direct
from Malay, though this will not be separately described since it differed in no way from
the form of the species available at Kew. It will be convenient to deal with the genera
in the order listed above, and the species in the genera in the order of their geographical
distribution.
All the available species of Osmunda are represented in Fig. 277 except for 0. regalis,
which can be seen on reference back to Fig. 22 a, p. 36. The chromosomes of the two
North American species, 0. cinnamomea L. and 0. Claytoniana L. ( = 0. interrupta) are
shown in Fig. 277 a and c. Two species or forms closely resembling 0. regalis but smaller,
276
ippHii3mnwsr#f .•.-:- -.j^i
•
*
t ■
,f
*■ »
...*
hie
v^'^.-^-
-s**
J-
f^ - . - • _ i.. „ -
iiM?'
t»»
/
IIP
r
%
v
t
o
%
n
•«f <k
* ; 1
^
*• «
%
P -
0 .
•►
-**
.»
*^
V
«r
Fig. 277. Meiosis in species of Osmunda. x 1000. a in fi-esh acetocarmine, all the others in balsam.
n= 22 throughout, a. 0. cinnamomea h. b. 0. gracilis Hort. c. 0. Claytoniatia h. d. 0. palustris
Hort. e. O.javanicaBl.
277
THE ANCIENT FERNS
and believed to have come in the first instance from South America, are 0. gracilis Hort.
and 0. palustris Hort.* (Fig. 276), the first a hardy deciduous plant and the second a
stove evergreen. Their chromosomes are visible in Fig. 2771^ and d. Lastly, O.javanica
J
^' ^ft.
>
%
A
0^
1»
«
Vr
\^ V
5
^ •^1% ■ '*^
-^
M '
.^
m^
1^
4C.
*te mm
W ^
^/.li33K^^^8
Fig. 278. Meiosis in Todea and Leptopteris, in permanent acetocarmine. x 1000. ;z=22 throughout.
a. Todea barbara (L.) Moore, b. Leptopteris Frazeri (Hk. et Grev.) PresL c. L. hymenophylloides
(A. Rich.) PresL d. L. siiperba (Col.) PresL
BL, the most genuinely tropical species, will be found in Fig. 277^. It is obvious at a
glance that all these species are very similar cytologically. All have a chromosome
number which is identical with that of 0. regalis, namely, n = 22. The only detectable
difference to the eye is indeed the slightly smaller chromosome size in the last
* The origin of this well-known horticultural plant is somewhat obscure, since it seems to have been
confused at a very early date with 0. gracilis Link with which it is not identical. If it does not come from
South America its origin is unknown.
278
THE ANCIENT FERNS
three species and the rather more diffuse outhne which seems to be characteristic of
0. javanica.
The available species of Todea and Leptopteris are grouped together in Fig. 278. Todea
barbara (L.) Moore is in Fig. 278^. The other three figures are of three species of
Leptopteris, namely, L. Frazeri (Hk. et Grev.) Presl, L. hymenophylloides (A. Rich.) Presl,
and L. superba (Col.) Presl. The close agreement of all these species also, both with each
other and with Osmunda, is very evident. Again, they all have n = 22 as regards
number, and the only detectable difference in this case is a slightly larger size of the
chromosomes in Leptopteris.
That the resemblance between the Osmundaceous genera concerns not only their
chromosome number but also some features of their chromosome structure is indicated
by the few observations on spiral structure with which this chapter may conclude. In
Fig. 279. Spiral structure in three genera of Osmundaceae. x 3000. In each case two chromosomes
from one species are shown, a. Todea barbara (L.) Moore, b. Osmunda gracilis Hort. c. Lepto-
pteris superba (Col.) Presl.
Fig. 279 a there is Todea barbara, in Fig. 279^ Osmunda gracilis and in Fig. 279c we have
Leptopteris superba, all differing characteristically in chromosome size according to the
specific differences already noted but alike to a striking degree in the pitch of coil, the
number of gyres per chromosome and so on.
One is left with the impression that the Osmundaceae alone, of all the ferns con-
sidered, are a genuinely primitive group which has remained primitive because it
has changed very little since a remote geological period. It is not merely a case of a
resemblance to an archaic ancestor shown by a form which has become specialized. The
primitive character is expressed in morphology, in anatomy and in the relatively simple
chromosome complement. This may perhaps suggest that the longevity and stabihty of
the family may in part be the expression of the stabihty of its nuclear construction.
This idea will be touched on again in the next chapter. In the meanwhile we may
offer one last comment on the possible significance of the chromosome number itself In
the Flowering Plants, as explained in Chapter 2, the commonest haploid numbers are of
the order 7, 8, 9 and 1 1, though unexpected jumps to 22 occur in several places in the
279
THE ANCIENT FERNS
Cruciferae and some other numbers can be found. In the Pteridophyta such low num-
bers have very rarely been encountered, but instead we find polyploid series on 37 or 41,
prime numbers which cannot possibly be primitive. Only in the microphyllous groups,
though in most of these indirectly, are we sometimes led to consider 9 or 10 or their
multiples. In the ancient ferns, however, there are signs of these numbers again.
Ophioglossum, as we have seen, may be polyploid on 8, Hymenophjllum and Trichomanes bear
a quite definite relation to 9, and the Osmundaceae are similarly related to either 1 1 or
22 as the case may be. The importance of these groups lies at least as much in this fact
as in the interest which they excite for their own sakes. No matter how imperfectly,
they demonstrate with sufficient clearness that even the Pteridophyta must have started
life in the distant past from cytologically simple beginnings. And before a general
interpretation of our data can be even attempted this is a fact which we very much need
to know.
SUMMARY
The Ophioglossaceae, Hymenophyllaceae and Osmundaceae have been examined, the
first two in their British representatives only, and the last also from botanic gardens.
Ophioglossum is remarkable as the highest chromosome number yet recorded in the plant
kingdom. Hymenophyllum is remarkable for having the largest chromosomes in the ferns
and the lowest chromosome number known in the group. The Osmundaceae stand out
as the only group which seems to be genuinely primitive in all respects.
2n
List of new chromosome numbers introduced in the chapter
Ophioglossaceae
Ophioglossum lusitanicum L.
0. vulgatum L.
Botrychium lunaria (L.) Sw.
Hymenophyllaceae
Trichomanes radicans Sw.
Hymenophyllum unilaterale Bory
H. tunbridgense (L.) Sm.
Osmundaceae
Osmunda regalis L. (Chapter 3)
0. cinnamomea L.
0. Claytoniana L.
0. gracilis Hort.
O. palustris Hort.
O. javanica Bl.
Todea barbara (L.) Moore
Leptopteris Frazeri (Hk. et Grev.) Presl
L. hymenophylloides (A. Rich.) Presl
L. superba (Col.) Presl
a
«
125-130
250-260
•
45
•
72
18
13
44
66
88
22
Irregular
44
22
22
22
22
22
22
22
22
22
280
CHAPTER 17
CONCLUSIONS
The facts are now before us and though minor conclusions have been drawn as they
arose in chapter after chapter it is perhaps worth the attempt to stand back a little to
survey the whole before ending. The survey will be partial and incomplete as are the
facts to be summarized but within the multiplicity of details which have at times
perhaps seemed over elaborate a kernel of continuity can nevertheless be extracted
which may be worth looking for. Before doing so, however, it will repay us to forget
for a moment the microphyllous and ancient groups which have occupied the last few
chapters and to think again of the more modern ferns dealt with earlier in the book, in
order to consider a little more closely the British fern flora as a sample of the world's
vegetation.
As we have seen, there are some 47 species of leptosporangiate ferns in the British
Isles, which were analysed in detail in Chapters 4-8. Polyploidy and hybridization
were met with so abundantly that we were at once able to conclude that the Fili-
cales have indeed utilized the same types of evolutionary mechanism as the Flowering
Plants. As the narrative unfolded in species after species, another impression was also
conveyed, namely that many of the polyploid changes encountered seemed to be of
relatively recent date and sometimes only partly fulfilled. Time after time, e.g. in
Polypodium, Cystopteris, Asplenium, Dryopteris, we were forced to discriminate, often with
difficulty, between populations in various grades of polyploidy and others, of lower
chromosome number, still present in the same geographical area. In many cases the
lineal relationship between low-numbered and high-numbered forms could be clearly
demonstrated though, in some, the low-numbered forms were either imperfectly known
or missing. Throughout, the general impression was gained as of a wave (or epidemic)
of polyploidy which has affected the flora as a whole and which has recently resulted
in the partial replacement of low-numbered species by their higher-numbered de-
scendants, a process which is perhaps still continuing. This confronts us at once with
the question as to whether the British fern flora is to be regarded as a fair and repre-
sentative sample of the ferns of the world or whether it is in fact only typical of the
vegetation of this part of Europe.
This is where a statistical comparison with the Flowering Plants is perhaps of
importance. As was foreshadowed by Tischler (1935) and more recently worked out
in greater detail by Love and Love (1943 and 1948) there exists a statistically significant
correlation between latitude and frequency of polyploidy if comparisons are made
within the rather limited area of Western Europe.* From this, Love and Love conclude
that polyploidy as such is an adaptation to cold.
* The very fragmentary information quoted by these authors from Sicily and Timbuctoo must here
be discounted as too unreUable to bear the weight of statistical comparison.
281
CONCLUSIONS
Table 8. Statistical frequency of polyploidy among Flowering Plants in different
regions of north-western Europe, after Love and Love, 1943
Percentage
Percentage of poh
I'ploids
of flora
A
r
Km. 2
Latitude
determined
Monocots
Dicots
Total
Schleswig-
20,000
54-55° N.
92-0
63-1
43-5
48-7
Holstein
Denmark
44,000
54^-58° N.
87-7
70-1
45-8
52-1
Sweden
449,000
55t-69° N.
8i-5
74-6
45-9
53-7
Nonvay
323,000
58-71° N.
83-6
74-3
45-9
54-1
Finland
383,000
60-70° N.
80-7
77-4
45-4
54-6
Faeroes
1,400
c. 62° N.
85-0
76-1
51-7
60-7
Iceland
106,000
63i-66i° N.
8o-o
84-3
53-4
63-5
Spitz bergen
63,000
77-81° N.
—
95-2
68-4
77-4
Table 9. Statistical frequency of polyploidy among the leptosporangiate ferns of the
British Isles and Madeira
British Isles
Madeira
Km.2
316,000
635
Latitude
50-61° N.
32° N.
Percentage
of flora f
determined Diploids
Total number of
Polyploids
100
91
22
22
25
16
Percentage
polyploidy
53
42
Leaving this last conclusion aside for a moment we may compare the British ferns
with Love and Love's data for Flowering Plants. These are summarized in Table 8, in
the form presented in their 1943 paper which for the present purpose is the more
convenient to quote. The top part of Table 9 gives the comparable facts for the British
ferns compiled with the sole assumption that the three cases for which only non-
British specimens have been available {Cystopteris montana, diploid D. dilatata, Asplenium
Adiantum-nigrum var. acutum) will actually be found here in the form predicted. With
this assumption, the total frequency of polyploidy among British ferns is of the order
of 50 per cent, and though the exact figure obtained (Table 9) should not be taken too
seriously since it might easily have been slightly different had the search for relict
diploids been less thorough, or had others been found. As an order of magnitude it
nevertheless compares closely with Love and Love's estimate for Flowering Plants in
general and for Dicotyledons in particular at comparable latitudes in Scandinavia.
Does this, however, mean that polyploidy as such is geographically determined either
in ferns or Flowering Plants as an adaptive response to cold? I personally think that
it does not, but rather that the historical incidence of recent glaciation has produced
this appearance locally and incidentally. If comparisons are conducted not within the
glaciated territories but outside them a somewhat different picture emerges which,
when it can be more fully seen, will almost certainly solve this particular problem.
The island of Madeira is an excellent starting point for such a comparison and though
the facts can at present only be quoted in a preliminary form they will be briefly
introduced at this point because they provide a valuable corrective against the hasty
acceptance of what may prove to be an incomplete, and therefore misleading, explana-
tion.
282
CONCLUSIONS
The fern flora of Madeira contains 42 known species of leptosporangiate ferns,
38 of which have been studied cytologically at the time of writing (Manton, unpubHshed) .
Madeira itself is a mountainous island off the coast of Africa placed in a subtropical
latitude quite outside the area of glaciation and which has therefore enjoyed a vegeta-
tion-cover without interruption since it emerged from the sea in Tertiary times. Of
its 42 species of ferns 22 have been attributed to species also present in Britain, the
remainder being species characteristic of warmer, in some cases of tropical, climates.
A few are endemic.
If now the British species are examined in Madeira, Love and Love's generalization
is borne out since out of 22 species only about 7 are polyploid, i.e. one-third
instead of half the relevant part of the flora. On the other hand, if we include the
non-British species as well, the total incidence of polyploidy though slightly less than
in Britain (Table 9) has another, perhaps even more significant, diff'erence, which is
not shown in the Table. In Britain (the Ophioglossaceae apart) the grade of polyploidy
is in most cases that of tetraploid with only two cases of hexaploids in Cystopteris and
Polypodium respectively. In Madeira on the other hand, among the non-British species
so far analysed there are two octoploids and a decaploid clearly recognizable as such
by the fact that they belong to well-known British genera: the species in question
are Asplenium aethiopicum (Burm.) Bech. {=A. furcatum Thunb.) n=i^/^ (octoploid),
Polystichum falcinellum (Sw.) Pr. ^=164 (octoploid) and Adiantum reniforme L. ^=150
(decaploid). Further, while in Britain the polyploids are in most cases still in geo-
graphical contact with lower-numbered relatives (from which circumstance they are
thought to be recent), in Madeira these high chromosome numbers all belong to
undoubtedly ancient and isolated types totally devoid of local relatives and in
some cases, notably Polystichum falcinellum and Adiantum reniforme, either endemics
or nearly so. These must therefore be ancient species with a long past history
which, for this reason, is no longer spread before us as in so many of our own native
species.
My personal conclusion from this is that polyploidy as such is not in itself either
ancient or modern or an adaptation to cold or any other single climatic or ecological
factor but that it is correlated rather with climatic or geographical upheavals however
caused. Under stable conditions, the natural spread of species is probably accompanied
by some, though perhaps infrequent, polyploidy as new species come into contact with
old ones and hybridize with them. In a relatively undisturbed flora the incidence of
polyploidy might therefore be expected to be low. Under changing climates or topo-
graphy, on the other hand, the opportunities for hybridization and therefore for
allopolyploidy can hardly fail to be increased. The nature of the significant climatic or
topographical changes may be very varied and include perhaps cold, heat, drought,
inundation, mountain building, volcanic action, changes in the distribution of land and
sea or any other vicissitude which may affect the whole earth or portions of it. All of
these may be expected to leave their mark on the evolution of vegetation.
If this is so, the high polyploids of Madeira may perhaps record world events of a
different type and older than the Ice Age whereas the polyploids of Britain and of
north-western Europe in general may still bear in their distribution and physiological
283
r
CONCLUSIONS
preferences the imprint of the most recent geological event which has engulfed this area,
namely a repeated succession of glacial and interglacial periods.
If this is even partially true we are forced to the conclusion that, in the statistical
attributes of their cytology, the British ferns are not a representative sample of the ferns
of the world but that in details they are typical only of the vegetation of Europe in this
particular age and latitude.
This need not, however, embarrass us unduly, for the microphyllous groups and
therefore the Pteridophyta as a whole can scarcely be subjected to statistical treatment
on our present limited knowledge, and discussion of them must therefore be confined to
those qualitative features which even a small or imperfect sample displays quite clearly.
Leaving this digression on the British ferns aside, we may therefore now proceed to
a more general discussion of the larger group of which the Filicales, though a part, are
only one among many. Lest, however, the reader should at this point expect the
impossible and look for a general discussion of all the varied aspects of a complex and
rapidly growing subject in a manner proper only to our imaginary historian of Chapter i ,
it may be well to recall the rather limited terms of reference defined at the beginning of
Chapter 2 and within which the inquiry has been conducted. At the present stage it
would hinder and not help us to attempt to equate the results arrived at with all the
current views on general evolutionary topics which have been voiced from time to time
by students of other groups of plants or of animals. The remainder of this chapter will
therefore contain only a summary discussion of the facts presented in the book itself
and of the conclusions directly arising from them. The further evaluation of these, and
in particular their assimilation into the general body of knowledge which is mainly
based on the Flowering Plants, although it must eventually be attempted, would require
far more than the end of a chapter and may fittingly be left to the future.
The first general conclusion from the work as a whole is perhaps the justification of the
method. Cytogenetics when applied with care and with modern techniques is at least
as informative in the Pteridophyta as in any other group of plants, and it is quite
certain that important new light will be shed on many hitherto insoluble problems of
taxonomy and of phylogeny when further work has been done. It may, however, be
well to warn a beginner yet once more against over-confidence and the hope of quick
results. The problems are legion and may be pursued in almost any country, but they
cannot be solved quickly, and unhmited care must be urged upon any would-be
investigator in the verification of specimens, in the recording of their places of origin
and in the full authentication of cytological observations, or, in a group as difficult as
this is technically, great harm may be done by the perpetuation of the types of error
which have dogged investigators with very few exceptions in the past and which once
recorded in print may be very hard to eradicate.
A second conclusion is that the Pteridophyta as a whole while employing many of
the same evolutionary mechanisms as those of the Flowering Plants, have in some
respects proceeded further than the Flowering Plants, as their longer history had led
us to expect. The very high chromosome numbers recorded in every major group are
likely in themselves to be a sign of antiquity, though the continued presence, even in
the most ancient groups, of some genera and species with low numbers seems to
284
CONCLUSIONS
indicate that in the distant past the cytological state of the Pteridophyta as a whole must
have been more hke that of the Flowering Plants of to-day than is now the case.
Of the various types of mechanism enumerated in Chapter 2, all have been seen to be
operative, though the degree of completeness in our knowledge about them varies very
greatly, as was to be expected. It may be of interest to pass each briefly in review.
(i) Hybridization has been met with unexpectedly often and in a great variety of
groups. Outstanding examples were Equisetum litorale, trachyodon and Moorei, perhaps
Lycopodium Selago, and the various ferns, the latter ranging from frequently reformed
hybrids such as Asplenium germanicum, Polystichum illyricum, Dryopteris uliginosa and so on,
to very ancient hybrids now ranking as species owing to their loss of sexual reproduction,
such as Pteris cretica and the other apogamous ferns.
(2) Polyploidy is present in almost bewildering profusion and has reached levels not
yet touched by any other group of plants. It is only necessary to recall the 205 chromo-
somes of pentaploid Dryopteris Borreri, the approximately 204 of tetraploid Psilotum, the
216 of Equisetum (the sporophytic and not the reduced numbers are, of course, now being
quoted), the 222 of hexaploid Poly podium, the 252 of hexaploid Cystopteris, the 400 odd of
Tmesipteris, the 500 odd of Ophioglossum vulgatum, to realize how much further these
nuclear processes have gone here than in the mere 81 or 120 chromosomes which con-
stituted the maximum numbers quotable in the Cruciferae. The most probable con-
clusion to draw from this fact is, as already suggested, that high chromosome numbers
are not primitive but a sign of antiquity. The undoubted tendency for outstandingly
high numbers to accumulate most conspicuously in the most ancient groups (Psilotales,
some Lycopods, Equisetum, Ophioglossum) will then become intelhgible and may further
be found to denote a measure of senility in these groups.
It is not always possible to assess with certainty the grade of polyploidy involved. In
favourable cases where the monoploid state is a prime number, as in Dryopteris (« = 41) or
Polypodium {n = 37), we may diagnose, say, hexaploidy with certainty, but in many other
instances we are clearly deahng with only the upper members of series whose bases have
been lost. In these we can infer the existence of polyploidy and some facts about it only
by the indirect evidence of the arithmetical attributes of the numbers {Cystopteris,
Ophioglossum), or the uncertain evidence of comparisons external to the groups
(Equisetum) . By all these means, however, a strong suspicion is raised that the rather
rigid hmitations on nuclear increase encountered in the experimentally induced auto-
polyploid series of Osmunda, which ended in sterility at ^n, and in similar series in other
parts of the plant kingdom (see Chapter 3), has been far exceeded by the degree of
polyploidy actually achieved in nature. The reason for this is uncertain, but it may at
least suggest to us that there are some effects of longevity which are not exactly
reproduced when these processes are imitated in the laboratory.
With regard to the type of polyploidy it is noticeable how little sign of autopolyploidy
has been discovered except in the artificially induced series of Osmunda (Chapter 3).
Psilotum is the only clear case in which simple autopolyploidy is suspected on positive
grounds. It should, however, be remembered that the only positive criterion for the
diagnosis of autopolyploidy is multivalent pairing, and we do not know for certain
whether in the course of thousands or millions of years this power might not become lost.
285
CONCLUSIONS
(3) Allopolyploidy, i.e. polyploidy imposed upon hybridity, is surprisingly common,
and although, in the Pteridophyta, the best authenticated examples such as the Male
Fern or Polystichum aculeatum have all so far been found among ferns, it is most unlikely
that the phenomenon is confined to these but rather that full demonstration in the
microphyllous groups is more difficult. In the Polypodiaceae, however, as we have seen,
the evidence from the British flora is sufficiently advanced to permit, for the first time,
of a statistical estimate of frequency. Within the 50 per cent of polyploids which
characterize the whole fern flora (Table 9) we have diagnosed or given reasons to sus-
pect a hybrid origin in at least eleven cases, i.e. D. Filix-mas, Polystichum aculeatum,
perhaps the ancestor of the D. spinulosa or D. dilatata complexes, perhaps D. Villarsii,
at least one and possibly more species in each of Asplenium, Cystopteris and Polypodium,
and perhaps Woodsia alpina, Phegopteris and Dryopteris Borreri. This number is almost
certainly an underestimate but it amounts to about a quarter of the total flora.
(4) Aneuploidy is also very conspicuous though less frequent. As in the Cruciferae it
tends to characterize the relation between genera or groups of genera rather than be-
tween species. A partial exception is at first sight suggested by Isoetes and Lycopodium
(see Chapter 15), though the true meaning of the facts here may merely be that the
greater antiquity of the Lycopods relative to the ferns has resulted in a shift in the
phyletic value of the systematic units named by taxonomists, so that a species in, for
example, Lycopodium, is really the equivalent of a genus among ferns. As in the Cruci-
ferae the numerical order of chromosome numbers involved in effective aneuploid
changes is far lower than those involved in polyploidy, though the actual numbers are
considerably above those of the Cruciferae. The commonest monoploid numbers among
ferns fall (Chapters 4-1 1) between 29 and 41, as opposed to 6 to 1 1 in the dicotyledonous
family, again perhaps the result of antiquity.
(5) Genie mutations must also be important, perhaps indeed of primary importance,
since they are presumably involved as a principal factor in the formation of many, if not
all, of the residual half or three-quarters of total species in which allopolyploidy is not
involved. It may, indeed, be suspected that a most essential clue to the causal aspects
of evolutionary mechanism must lie in the analysis of the nature of interspecific diflfer-
ences between species in which no gross cytological differences can be detected. In the
Pteridophyta this subject is at present a closed book, and it is perhaps the largest topic
in which the study of this group at present lags greatly behind what has been learnt
from some of the more favourable Flowering Plants.
(6) Another subject on which our knowledge is deficient is the phyletic significance
of chromosome shape. In the course of this book spiral structure of chromosomes has
been demonstrated incidentally in Hymenophyllum, Equisetum, Psilotum, Todea and
Leptopteris in addition to Osmunda in which it was already known, thereby increasing the
comparative interest of the Pteridophyta as cytological objects. The peculiarities of
texture encountered in the chromosomes of many of the microphyllous groups which
result in their very unusual shapes is, however, a new phenomenon, and one which well
deserves further study from the point of view of pure cytology.
Changes of chromosome shape are less easily observed in this group than in many
others, though the genus Hymenophyllum would be admirably suited to their study if more
286
CONCLUSIONS
species could be investigated and especially if breeding work could be carried out.
Here, if anywhere in the Pteridophyta, we might hope to learn something about the
mechanism involved in aneuploid changes.
(7) Chromosome size was commented upon in Chapter 16, but the possible signi-
ficance oUhanges o{ ?>\x&, especially in association with advancing polyploidy, should per-
haps be listed here. It can frequently be observed {Ophioglossum is a good example) that
if related species are compared, the one with the higher number will often possess the
smaller individual chromosomes and the reverse has not been found. This suggests
that diminution of chromosome size must often either accompany or follow the inci-
dence of polyploidy, and since our experience with Osmunda gave no indication of the
former, it seems possible that the order of events is the latter. The interest of this obser-
vation is in its possible relation to the paradox enumerated under item (2) above,
namely, that the grades of polyploidy occurring naturally seem incommensurate with
the limits encountered in artificial series. It seems probable that some nuclear or
physiological readjustments must occur with the passage of time to restore the power of
an organism to sustain a repetition of polyploidy on a scale which would be impossible
otherwise. We know nothing of the nature of such readjustments, but diminution of
chromosome size is perhaps one.
This enumeration of evolutionary mechanisms expresses the factual basis for the com-
parison of the Pteridophyta with the Cruciferae, though it does not wholly exhaust the
general conclusions which can be drawn, some of which will next be discussed.
An observation which is strengthened by the facts in both groups is the difference in
evolutionary effect of aneuploid and polyploid changes. The latter make species only.
The former make species also in the first instance, but such species seem usually to be
potential genera or larger groups, since they have not been encountered except in their
descendants which are thus designated. Why this should be so is by no means self-
evident, though we may suspect that reproductive isolation of a more effective kind
than is achieved by polyploidy may have something to do with it (cf. Manton, 1932),
though this cannot be the whole story. The fact, however, forces us to reahze that the
fate of a species may depend as much on its method of origin as on any other circum-
stance, a conclusion which is perhaps in itself of some importance.
An observation which emerges far more clearly from the Pteridophyta than from the
Cruciferae is the apparently high survival value of the high chromosome numbers. Their
accumulation in the most ancient groups was not unexpected, but the tendency of the
low-numbered forms to die out first is not so easily explained. That this is the position
is, however, suggested in group after group. It is only necessary to recall the numerous
cases among ferns {Cystopteris, Polypodium, Dryopteris, Asplenium) in which polyploids are
abundant, but the related diploids have to be looked for with care, to be satisfied that a
wave of polyploidy is affecting our flora, which may or may not be exceptional and an
eff^ect of recent glaciation as discussed on p. 283, but which now gives the impression
of a recent replacement of older, low-numbered, species by newer descendants of higher
chromosome number. There are exceptions of course, but the really glaring exceptions
such as Selaginella (^ = 9), terrestrial Isoetes («= 10), Hymenophyllum (n= 13 and 18) and
Osmunda {n = 22), stand out as much by the relative absence of polyploidy as by
287
CONCLUSIONS
the persistence of low chromosome numbers, and they only serve to enhance the
generality of the rule that where polyploidy is extensively practised, the low-numbered
species are the most likely to die out first. This, as we have seen, may mean that in a
really ancient group such as Equisetum, the Psilotales or Ophioglossum, the origin of a
series may become effaced, and it can then only be reconstructed in imagination from
indirect evidence.
An unequivocal reason for this behaviour is not easy to diagnose, though suggestions
about it can of course be made. The mutational character of allopolyploid species,
i.e. their sudden appearance, may have something to do with it. This may perhaps be
specially important if, as must often have happened, the two parent species have been
brought into juxtaposition by some unusual circumstance such as changing climate,
which may destroy old habitats together with their inhabitants and liberate new ecologi-
cal sites for colonization, provided only that suitable colonists, not too closely bound to
the older conditions, can step in while the opportunity is open. Under these circum-
stances, the possible advantages of mutational change (saltation in the sense of the old
mutation theory), as opposed to the slower process of adjustment to changing conditions
by means of natural selection acting on the raw materials (e.g. polygenes, major genes
or recombinations of biotypes) provided by genie mutations, are obvious, and the effect
may well be decisive in determining which forms can survive. Another possible
explanation for the replacement of low- by high-numbered species is hybrid vigour in
the latter; but this is almost certainly to be discounted in view of the immense lapses
of time which must be involved in the changes which we are considering.
That to a limited extent diminution of chromosome number can also occur is suggested
in the Pteridophyta by the solitary evidence o{ Hymenophyllum (Chapter i6) and perhaps
Doodia (Chapter 12). With fuller knowledge other and clearer examples would almost
certainly be found. These would, however, in no way disturb the generality of the
rule that in this group the aneuploid changes no less than the polyploid ones, though
at a slower rate, tend on balance to inerease chromosome numbers if a long enough
period of time is considered. This observation could not have been made on the Cruci-
ferae alone, and it is indeed possible that the Flowering Plants may respond to old age
differently. If they do not, or if this phenomenon is at all widespread, we must
recognize it as one at least of the possibly numerous factors which lead ultimately to
th^ decay of once dominant groups.
To pursue this idea further, we may survey the Pteridophyta and ask ourselves where,
if anywhere, could a new great group of the future arise ; but we should be at a loss for
an answer. If we have diagnosed the evidence aright, that all great groups must start
from simple beginnings with low chromosome numbers, the choice is limited. We have
Selaginella, Isoetes hystrix, Hymenophyllum and Osmunda. Only an irrepressible optimist
would, however, expect big developments now from such peculiar and apparently
stereotyped and specialized forms as the first three, and Osmunda, though probably still
primitive, is known to have had its biggest burst of macroevolution in the very distant
past, and all the fossil evidence we possess gives no instance of a recrudescence of effective
evolutionary activity after so great a lapse of time except perhaps of the 'gerontic' sort,
i.e. as a senile outburst preceding extinction. We seem forced therefore to the con-
288
CONCLUSIONS
elusion that the evolutionary potential of the Pteridophyta as a source of major
innovations into the earth's flora is running down.
The reasons for this seem very varied even on the flimsy evidence we have. High
specialization is known to check evolutionary potential in other groups, and this is per-
haps the condition in Isoetes and Hymenophyllum. Selaginella appears to be stereotyped,
perhaps from structural reasons, for among the several hundred species which differ in
details, none is sufficiently distinct to tempt taxonomists to call it a genus. Selaginella
now seems incapable of any form of macroevolution and has probably been in this con-
dition for millions of years, even if it was not always so.
That cytological conditions in themselves can be factors which lead to a slowing down
of evolutionary activity is, however, a conclusion which more particularly emerges from
the present study and which, if true, is perhaps one of the more valuable parts of it. In
the Osmundaceae we have already remarked upon the coincidence of a primitive
morphology and extreme uniformity of nuclear structure among the modern genera.
The cytological condition here seems to be primitive, and it is perhaps also relevant to
recall the marked difficulty recorded in Chapter 3 of obtaining viable sporophytes with
any considerable departure from the normal complement of chromosomes. Another
observation, well known to gardeners, is the great scarcity of mutant forms of even much-
used ornamental plants like Osmunda regalis in contrast to the profusion of monstrosities
which most other British species have yielded. We seem here to be dealing with an
ancient group in which the nuclear constitution has, for unknown reasons, become so
stable that it is now almost incapable of change either genetically or cytologically. To
this fact the long retention of primitive morphological characters may perhaps be due,
and the success of such species as we have, over such very prolonged periods of time, may
perhaps be attributed more to the physiological resilience which makes them so
tenacious of hfe under very varied conditions of climate or culture, than to any
specially adaptive features in their morphological characters as such.
That high chromosome number in itself may act as an internal factor tending to
slow down evolution is strongly suggested by a comparison between the two almost
equally ancient genera oi^ Selaginella and Equisetum. In the one, Selaginella, we have 800
taxonomic species. In the other, Equisetum, a genus which in temperate latitudes is at
least as successful if area of ground occupied is a valid guide, there are only some two
dozen species on the whole earth's surface. Both have existed for milUons of years, and
should have had ample time to develop their full potentiahties. Is the difference be-
tween them perhaps in part due to the fact that genetical innovations become effective
more easily where the haploid chromosome number is 9 than when the nucleus has be-
come loaded up as it were, by aneuploidy or by polyploidy, to a prevailing gametic
number of 108? If this is so, we may include high chromosome number among the factors
which lead to evolutionary stagnation, and see in it perhaps one of the commonest
reasons why so many of these ancient groups have remained primitive in spite of a
tenacity to life which has ensured their survival. We may also predict that the Fihcales,
that large and lively group from which so much of our evidence has been derived, are
tending that way and are almost certain, in the future, to become engulfed in their
own increasing complexity and probably to succumb, except for their hardiest or
Mpc 289 ^9
CONCLUSIONS
luckiest descendants, as the inevitable consequence of the working of their internal
evolutionary machinery.
This is a highly mechanistic conception which disregards the actual morphology of the
plants themselves and also their degree of structural 'adaptation' to their mode of life,
not from any prejudice against the Darwinian or other theories of evolution but by the
force of logic in the facts before us. Adaptation there must clearly be or a plant cannot
survive, and if an ecological niche suitable for such survival is not available at the right
time a potential new species will not become established or an old one will die out. But
to look here for the mainspring of Macroevolution seems to me personally a fruitless
quest. That once established, most species have a very considerable power of physio-
logical adjustment to their environment by Microevolution has been proved in the
Flowering Plants by the work on experimental ecology initiated by Turesson (1922)
and powerfully amplified among others by Clausen, Keck and Hiesey (i 940-1 948) in
their admirable Experimental Studies in the Mature of Species. Such work has not yet been
extended to the Pteridophyta and it is very desirable that it should be. It shows
unmistakably that it is possible and indeed probable that th^ pace of evolution may be
accelerated or retarded by environmental pressure, since the greater the opportunity
the more frequently will new forms become established, and, conversely, the more
extreme the changes in a given environment the greater will be the number of species
to die out. Great evolutionary activity may therefore accompany or follow periods of
violent climatic oscillations, such as an ice age. But that the major evolutionary trends
have been primarily caused by such changes seems impossible to believe. We have been
studying, in the Pteridophyta, an ancient group which has survived innumerable
cosmic vicissitudes and which had already become subdivided almost at the dawn of
the fossil record into the various main branches which still survive, together with others
which have died out. In our study of species we have encountered several examples of
parallel evolution and a great many examples of the production of new forms by
mechanical processes such as hybridization, polyploidy and so on. At the same time,
in studying the major groups as a whole, we have in the last few paragraphs had
repeatedly to call attention to a variety of different mechanisms by means of which
evolution seems to have been slowed down or stopped. At no point have we been con-
strained to look outside the organism for a directive influence.
This is perhaps the one point at which serious comment on generalized evolutionary
theories may perhaps be offered, while observing the limitations on the scope of the
argument explained in earlier pages. In most general theories, from the time of Lamarck
almost to the present day, attention has primarily been directed to the search for some
mechanism either outside or inside a living plant or animal by means of which, in the
course of generations, its character, appearance and functions might become changed
in an orderly manner, and without which these might be presumed to remain unchanged.
We may now perhaps ask ourselves whether an avoidable difficulty may not have been
introduced into the quest by this last assumption, unconsciously accepted as it usually is.
The emphasis which we have repeatedly had to lay on the detection of mechanisms by
means of which evolution appears to have been stopped, may suggest that a more help-
ful basic assumption will perhaps be found to lie in the truism that evolution, as such, is
290
CONCLUSIONS
a phenomenon for which no cause need be assigned other than the fundamental
instabihty of hving matter.
The apparent orderhness of evolutionary progressions, when viewed from a distance
against the background of the geological time scale, may perhaps only express the fact
that at any one period, in a given organism with a given structure and development, only
a limited number of types of innovation are possible without prejudice to its efficiency
as an individual, but that such changes as can occur most easily will do so repeatedly.
The majority even of potentially successful innovations will be expected to disappear
without trace, together with the vastly greater number of intrinsically unsuccessful ones,
since the accident of opportunity must also coincide. Given long enough, however, an
inherent instability in certain directions rather than in others will express itself taxo-
nomically, as parallel evolution or an orthogenetic* trend. Both of these have been
encountered in the Pteridophyta. We need only recall the soral characters o{ Dryopteris
or Athyrium and the complex apparatus of obligate apogamy for examples to demon-
strate the repeated origin of similar innovations in different places. Even the polyphyletic
origin of a species is perfectly possible in certain cases, e.g. hexaploid Polypodium. On
the other hand, the polyploid series itself which has been met with so abundantly could
be thought of as an example of the stepwise increase of a character caused by the
repeated introduction of similar innovations in time, which could easily have a
morphological equivalent. If indeed, to make this suggestion more precise, we replace,
in imagination, apogamy by heterospory and advancing polyploidy by progressive
precocity of the gametophyte, we are almost within sight of macroevolution leading
to the seed habit, without seriously overstraining creduhty as to what might reasonably
be supposed to have happened.
That the polyploid series is a valid analogy, indeed perhaps even a special case, of
an orthogenetic trend, is further suggested by its apparent relevance to another observa-
tion, no less remarkable for being famihar, namely, the surprising tendency of biological
systems to change in the direction of increasing complexity. We are told by physicists
that one of the most fundamental statements of experience in the inorganic world is the
second law of thermodynamics which says that entropy, by which is meant randomness
or disorder, always tends to increase. Yet here before us in both plant and animal
kingdoms we find the most elaborate kinds of order spontaneously generating them-
selves, not indeed out of nothing, but from simpler beginnings, and this, even at the
price of ultimate extinction from over-elaboration. What is the reason for this, at first
sight, flagrant contradiction? That the energy of respiration is sufficient answer is
difficult to believe.
This may perhaps suggest that to understand evolution in general terms we need to
* Although the sense in which this word is used here will probably be clear from the context and from
what follows above, it should perhaps be emphasized that it does not imply the existence of any ' inscrutable
creative force' as is sometimes assumed (e.g. Sewall Wright, 1949). A useful definition may be quoted
from the glossary to the symposium on Genetics, Palaeontology and Evolution (Jepsen, Simpson & Mayr,
1949) which has reached me at the last moment before this manuscript goes to press and which cannot
unfortunately be discussed as a book, '...orthogenesis. Evolution continuously in a single direction over a
considerable length of time. Usage differs considerably, but the term usually carries the implication that the direction is
determined by some factor internal to the organism — '
291 ^92
CONCLUSIONS
look not outside but inside the organism and in particular to study, with all the new
tools foreshadowed in Chapter i, and no doubt many others besides, not merely the
external attributes of chromosomes (their numbers, shapes and homologies) as in this
book, but rather their intimate molecular structure. We have here the usual atomic
components of the inorganic world harnessed together in a manner which strikes a
physicist as unfamiliar. This has been eloquently expressed by Schroedinger in a little
book called What is Life? with which all biologists should be acquainted. We know
enough about chromosome structure to be certain that their remarkable power of
initiating and controlling development is due not to the statistically determined
behaviour of bulk matter but to the delicate adjustment of the spacial pattern of
a relatively small number of individual atoms in a molecular fabric. It is this fabric
which now needs to be studied to determine its properties and laws of behaviour. More-
over, once again this must be done objectively, without preconceptions derived from the
inorganic world. For though we may be certain that matter has not ceased to be
matter by becoming harnessed into a novel configuration, the consequences of such a
configuration must also be expected to be novel and must first be explored for their
own sake before they can be used to explain other phenomena.
The problem of evolution is thus only one aspect of a larger problem of life, of growth
and of reproduction. And if we could really know how an organism contrives both to
develop and to transmit its likeness with such surprising fidelity from generation to
generation we might after all have unravelled the greater mystery.
292
APPENDIX 1
NOTES ON THE CYTOLOGICAL TECHNIQUE
FIXATION
The best fixatives for sections of most members of the Pteridophyta are half-strength
chromacetic-formaHn for roots and 2BD for sporangia. Full-strength chromacetic-
formalin can also be used for sporangia and for these is better than the half strength.
The formulae of these fixatives are given below. Sporangia should always be momentarily
dipped in alcohol (70-90%) or acetic-alcohol as a preliminary to fixation, though care
should be taken not to carry over excess of alcohol into the aqueous fixative ; a con-
venient method is to dip a sorus or sporangium into spirit for just long enough to wet it,
then to put it momentarily in contact with a piece of fabric to absorb the surplus alcohol
before transferring quickly to the proper fixative. Very large sporangia such as those of
the Psilotales should be punctured before fixing.
Bleaching before staining is of course necessary after the osmic fixative. This can be done
in the usual way by leaving wax-free sections in diluted hydrogen peroxide overnight.
For squash preparations of every kind acetic-alcohol fixation is generally satisfactory.
The precise concentration is not critical, i : 3 glacial acetic acid : absolute alcohol is
correct for some genera, e.g. Polypodium, though for others i : 2 is sometimes better. In
cold weather, fixation of several days (3 days to a week) is required for successful carmine
mounts, though in the height of summer overnight is sufficient. For Feulgen squashes
10 min. to half an hour's fixation is often satisfactory.
FORMULAE OF AQ.UEOUS FIXATIVES
I. Chromacetic-formalin:
Solution A :
Chromic acid
I gm.
Water
65 c.c.
Glacial acetic acid
10 c.c.
Solution B:
Commercial formalin
40 c.c.
Water
35 c.c.
Mix in equal parts immediately before use. For the half-strength fixative dilute with
an equal volume of water after mixing.
II. 2BD (La Cour, 1931):
Chromic acid 1% 100 c.c.
Potassium bichromate 1% 100 c.c.
Saponin o-i g.
Osmic acid 2% 30 c.c.
Acetic acid 5% 30 c.c.
293
NOTES ON THE CYTOLOGICAL TECHNIQUE
Since osmic acid in solution is unstable a stock solution without this component was
generally made up. When required for use, i c.c. of 2% osmic added to 7-3 c.c. of stock
solution makes the complete fixative.
STAINING
For most sections of meiosis Heidenhain's haematoxylin used as described below was
successful, and with good fixation a counterstain with Bismark brown to show up cyto-
plasm and cell walls is a great improvement. For roots a more transparent stain is
needed wherever chromosome numbers are high, and for this purpose gentian violet was
satisfactory. With both these stains a difficulty very frequently met with in roots is the
presence of large amounts of some substance in the cytoplasm (perhaps tannin) which
holds the stain tenaciously and is liable to disfigure preparations, although, if this can
be tolerated, clear staining of the chromosomes even in heavily affected cells can be
obtained (see, for example, Fig. 187). This difficulty is less troublesome after osmic
fixation, but the chromosomes are as a rule easier to count after chromic fixation (in
roots) so that the disfigurement has usually to be accepted. The innermost layers of
roots are free from this difficulty but are generally less suitable for chromosome counts.
Feulgen staining was used very rarely with sectioned material (see Fig. ^id), though it
appeared to present no difficulty. With squash material this stain was sometimes bril-
liantly successful after acetic-alcohol fixation, notably in Equisetum and Ophioglossum. In
some other cases, notably meiosis in the Osmundaceae and Polypodiaceae, it failed
completely after this fixative and was not further investigated, since in most of these
species acetocarmine was found to be adequate. The schedule for acetocarmine and
Feulgen staining used will be described below under Squashes.
STAINING PROCEDURE WITH HAEMATOXYLIN
Mordant half an hour. Wash in running water for three-quarters of an hour. Stain
overnight. Rinse in running water (half-hour). Differentiate in alum. Wash for 4 hr.
Dehydrate, clear and mount. For details of the solutions and for the insertion of a counter
stain see next section.
Note. In places with hard water, preparations should probably be dipped in distilled
water before entering the stain. This, however, was not necessary in either Manchester
or Leeds, where the domestic tap water is exceedingly soft.
FORMULAE OF STAINS
Haematoxylin was prepared and used as advised by Dame Helen Gwynne-Vaughan. The
dry stain, supplied by Messrs Gurr of London, was dissolved in absolute alcohol in a 10%
concentration and left for at least 3 months. When required for use, 5 c.c. of this solution
is made up to 100 c.c. with distilled water; it is then stirred with a glass rod previously
dipped in mordanting alum and then left for 4 days. After this, staining is at first
somewhat faint, though preparations become brighter with keeping. The pot improves
considerably after a month of constant use, and it will then remain in good condition for
at least a year. The stock solution will remain in good condition for several years but
294
NOTES ON THE CYTOLOGICAL TECHNIQ.UE
probably not indefinitely. After lo years I have sometimes had to discard an old stock
owing to the formation of a chocolate-coloured precipitate when it was mixed with
water. In old stocks it is also sometimes better to dilute down to 1% instead of |% for
the working strength, though at this dilution the stain becomes exhausted more rapidly;
it may, however, give very bright and clear preparations while it lasts.
Alum. The correct strength of iron alum varies somewhat with the material, but
usually the Pteridophyta require much weaker alum than the Flowering Plants. A
2 % solution for mordanting and an 8 % solution for differentiating is often satisfactory.
(In Flowering Plants I have generally used the strengths in the reverse order.)
Bismark brown. A 2% solution in 90-95% alcohol is convenient. The counterstain can
then be introduced, after haematoxylin, last thing before finishing the dehydration.
Alternatively, the stain may be dissolved in a weaker alcohol (50 or 70%) and intro-
duced during the dehydration, a procedure which is sometimes preferred as leading to
better washing off of surplus stain. The length of time required in the stain varies with
the age of the solution but should be of the order of 2 min.
Acetocarmine. The only secret in this stain is to have one's solution strong enough.
Heat a 45% aqueous solution of glacial acetic acid to boiling-point with excess of
carmine. Cool and filter. Use distilled water and keep the vessel covered while
heating.
Schiff's reagent {Leuco-basic fuclisin) for Feulgen technique. Pour 100 c.c. boihng, distilled
water on to 0-5 g. of Gurr's basic fuchsin. Agitate thoroughly and cool to 50° C. Filter
and add 10 c.c. n/i -hydrochloric acid and 0-5 g. potassium metabisulphite. Shake well
and keep in the dark in a glass-stoppered bottle for 1 2- 1 8 hr . During this time the solution
bleaches to a pale straw colour and is then ready for use. Stored as above, the reagent
will keep in good condition for up to 3 months.
SQUASH METHODS
I. Simple acetocarmine. With large sporangia the very simplest of the squash methods may
be satisfactory. Thus in all the Osmundaceae 1 2 hr. fixing in acetic-alcohol followed by
breaking of a group of sporangia with a flat-ended needle into a drop of acetocarmine
will provide ideal material for squash preparations, without the need for any manual
pressure, provided that all the empty sporangia and other solid bodies are removed. It is
then only necessary to put on a cover-slip, conveniently a no. i thickness, | in. square,
and boil the preparation violently under one corner for a second or two, during which
process the cover-slip itself provides all the pressure which is needed. The initial size
of the drop of liquid should be such that after boihng the cover-slip clings closely to the
slide without air bubbles. In this condition it may be ringed with wax for further study
in the fresh condition or it may at once be made permanent. If this is to be done it
should be left for as long as possible, i.e. half an hour or until air begins to enter, before
lifting the cover-shp, to ensure the maximum adhesion of the squashed cells, and it may
be beneficial to the intensity of colour to heat gently several times, care being taken not
to reach boihng-point again or the cover-slip will merely blow off explosively and the
preparation be lost.
295
NOTES ON THE CYTOLOGICAL TECHNIQUE
II. Making the preparation permanent. The method used has been almost exactly that
originally devised by McChntock, though for ease of reference the details may be given
here as follows :
(i) Dissolve off the cover-slip by immersing the slide face upwards in 45 % acetic acid.
It is essential not to attempt to hurry this process by poking the cover-slip with a needle
or the more tightly attached cells will wash off.
(ii) Dehydrate by passing slide and cover-slip through graded mixtures of acetic acid
and absolute alcohol of the following strengths i : i, i : 3, i : 9, absolute.
(iii) Partially clear in i : i absolute and xylol from which it is safe to mount in
balsam. It is essential in mounting that the cover-shp should be replaced exactly over
the area from which it came. This may be clearly marked by the lines of dried carmine
formed round the edges of the cover-slip after the original boihng and which should on
no account be cleaned off until the preparation is finished, but if this is not sufficient it
may be convenient to mark the position of two corners of the cover-slip with a diamond
on the back of the shde before immersing it in the acetic acid.
III. Acetocarmine squash with manual pressure. With small sporangia in which it is im-
possible to remove the empty cases entirely additional pressure is needed. This applies
particularly to the Polypodiaceae with mixed sori, in which ripe spores which resist
pressure are always liable to be present on the same slide as the softer mother cells. The
spore mother cells are squashed out of their sporangia as thoroughly as possible with a
flat-ended needle into the stain and the larger lumps of tissue such as the indusia re-
moved. Not too many sori should be dealt with at one time, six or so on a slide are
generally enough. A cover-shp, which may be large or small according to taste, is then
put on and the preparation heated gently. Whether it should be boiled or not depends
on the intensity of colour and the softness of the material. If staining is faint, boiling
enhances it; on the other hand, if the cells are soft they may blow to pieces on boiling.
In either case the pressure which produces flattening of the cells is apphed by hand. The
warm preparation is put down on the bench, momentarily covered with a piece of
blotting paper and a finger passed rapidly over it with the precise degree of pressure
which can only be learned by experience. The field should then be searched at once,
preferably without ringing, and ruthlessly discarded if it does not show any specially
favourable cells. This is essential, because in the Polypodiaceae, where only 16 or some-
times 8 mother cells may be the entire contents of a sporangium, successfully squashed
and usable cells may occur singly on a slide and if not found at once be looked for in vain
afterwards.
As a routine procedure in deahng with this type of material every instance of an excep-
tionally perfect cell was photographed at once and perhaps also drawn while the pre-
paration was in the first wet condition, as a precaution against its loss on transfer to
balsam. A few such photographs have been included in this book (e.g. Figs. 183/^, 2i8(^).
In every case, however, the transfer to balsam by the method fisted in paragraph II
above was attempted, the only additional precaution needed being slow dehydration, a
delay of a quarter of an hour or longer in each of the alcohols being not too much. In
most cases a cell could be rephotographed in balsam and the greater number of the
figures in this book are of these ; occasionally the critical ceU became detached and lost
296
NOTES ON THE CYTOLOGICAL TECHNIQ.UE
or spoiled when the shde and cover-shp were first separated, and the benefit was then
reaped of the prehminary photographs.
IV. Modification of the acetocarmine technique. Orcine and Lacmoid were both tried as a
substitute for carmine (La Cour and Darhngton), though no benefit was felt. For the
Pteridophyta, carmine when properly used is very satisfactory, and the reagent is also
stable enough to remain in good condition for a year with only occasional filtering.
Lacmoid, under certain circumstances, had the disadvantage of rendering the chromo-
some brittle and was discarded for this reason.
Experiments were also made with alternative mountants such as 'Euparal', and
though satisfactory these showed no advantage over balsam and were therefore not
extensively employed. Euparal, indeed, has a disadvantage over balsam in that it is
soluble in the immersion fluid that was used (Leitz 'Objektol'), which precludes the
making of high-power observations near the edge of a slide. For this reason alone it
would have been discarded.
V. Feulgen squashes. These were only used sparingly as a check on certain troublesome
genera such as Equisetum, which had proved difficult to complete by other means. Both
in Equisetum and Ophioglossum very beautiful preparations were obtained by this means,
though the much smaller size of the individual chromosomes after this treatment than
when swollen by acetocarmine meant that little if any additional facts were learnt. The
chromosomes also became somewhat brittle and liable to separate at secondary con-
strictions.
The procedure adopted was as follows :
(i) Fix in acetic-alcohol for lo min. to half an hour.
(ii) Transfer to water.
(iii) Hydrolyse in normal HCl at 60° C. for 10-20 min. according to the material.
(iv) Transfer to Schiff 's reagent (see p. 295) in the dark for half to 2 hr.
(v) Transfer to 45% acetic acid on a slide, tease out, cover, warm gently and squash
gently under a piece of blotting paper.
(vi) Examine at once, and if successful transfer to balsam by McChntock's method
(p. 296 above).
Examples of this technique are reproduced in Figs. 218a and 220.
OBSERVING
There was nothing worthy of special note about the methods of observing exceot
perhaps the very close use of photography as an integral part of observation at
every stage (for further details see Appendix 2), and the liquid used as an immersion
fluid. The frequent necessity of making detailed observations on unringed liquid
mounts made the change from cedar oil as the immersion fluid to ' Objektol ' very advan-
tageous. Objektol (which is obtainable commercially from Messrs. Leitz of London)
does not stiffen on exposure to air and can be washed oflT with water. This means that
its presence is not a difficulty when slides have to be processed further, since it merely
disperses in the acetic acid, and being very liquid throughout the examination reduces
considerably the risk of damage to the slide by accidental movement of the cover-slip
which can occur when cedar oil has become stiff.
297
APPENDIX 2
NOTES ON THE PHOTOGRAPHIC TECHNIQ^UE
Though photography has been very extensively employed as an integral part of observa-
tion at every stage, there is nothing specially remarkable about the photographic
technique for straightforward observations such as those reproduced in half-tone
throughout the book. For photomicrographs of haematoxylin and gentian violet pre-
parations and also for natural-size photographs, Ilford Special Rapid Panchromatic
plates with appropriate colour filters were used. For carmine mounts some of the later
photographs were taken on Thin Film Half-Tone plates, since these give greater contrast
especially with a red object.
The methods used for the making of text-figures are perhaps less well known and the
following processes may be of interest:
(a) Photographic basis for drawings. A camera lucida was not used for any of the black
and white diagrams, but all were drawn on the basis of a photograph which in many
cases is the photograph reproduced. The original photographs were all taken at standard
magnifications of 500, 1000 or 2000 diameters, according to circumstances and from
them drawings were made according to the following procedure. An enlarged print at
a higher magnification (2000, 3000 or 4000) is made on to matt-surface bromide paper.
This is then inked over in the usual way, after which the photographic image is bleached
(see below) and the drawing alone remains. A very suitable paper for this purpose was
found in Kodak WSM. 3.S which has a very convenient texture both for drawing and
for subsequent treatment, but any ordinary matt paper can be used. The advantage of
tliis method of obtaining diagrams is that far greater accuracy is achieved than with a
camera lucida and with much less effort to the obseri^er; a thing of importance where
nuclei of the complexity of those shown here are to be analysed.
(b) Method of bleaching. Any normal photographic method can be used, but to avoid
the handhng of KCN, which is a dangerous reagent outside a chemical laboratory, the
following procedure may be recommended as having proved satisfactory.
A bleaching solution is prepared immediately before use by adding sufficient of a
stock solution of 10% potassium ferricyanide to the normal stock solution of 20% hypo
to produce a deep yellow colour. (The exact strength is not critical, but it is consider-
ably stronger than that used for normal reduction.) The print to be bleached is then
soaked in water until uniformly hmp and is then immersed in the bleaching solution
until the image has disappeared. The diagram which remains is then rinsed in water, care
being taken not to rub the ink, which is otherwise Hable to come off or run, and is then
given a brief bath in acid hypo to prevent or to remove yellow stains. It is then washed in
running water for half an hour. To assist drying it may be gently blotted. When dry
it may be necessary to touch up the ink here and there and the diagram is then ready.
(c) Duplication of drawings by means of a paper negative. If it is undesirable to use the
original drawing for subsequent purposes it is a very simple matter to obtain facsimiles
298
NOTES ON THE PHOTOGRAPHIC TECHNIQUE
which are virtually indistinguishable from originals. One method is to make a paper
negative by treating the drawing as a transparency and making a contact print of it
on to any slow contrasty printing paper. The two pieces of paper should be kept closely
in contact with the unexposed sensitive surface against the drawing either in a printing
frame or by other means. Illumination is through the back of the drawing, and a
negative print results, from which positives may be obtained by a repetition of the process.
Any ordinary gashght paper can be used, though Ilford Reflex Document paper no. 50
was found specially suitable and cheaper than gaslight paper for large diagrams.
(d) Duplication of drawings by reflex copying. If a drawing is much touched up, or has
writing on the back, or is on rather opaque paper or board, it may be inconvenient to
illuminate through it, and in that case reflex copying is to be preferred. The drawing is
placed face upwards and is covered by a piece of Ilford Reflex Document paper no. 50
with the sensitized side downwards. Close contact is essential, and may be obtained
either in a printing frame or by covering with a piece of glass held down by weights.
Illumination is through the back of the Reflex Document paper. The length of exposure
can easily be ascertained by trial, but an average duration is 20 sec. with a 60 W. bulb
at 2 ft. After development a paper negative is again obtained, from which positives may
be printed off" by normal contact methods.
(e) Silhouettes of fern leaves. A paper negative is made by putting the leaf in contact
with slow contrasty sensitized paper and exposing it as if for a photographic print. The
use of a printing frame is convenient if the specimen is small and ordinary contrasty
gaslight paper to be used. If the specimen is large or a part of a herbarium sheet which it
is inconvenient to disturb, Reflex Document paper may be preferred, placed either above
or below the specimen (cf. (c) and (d) above). The negative so obtained may be used
for printing off" positives in the usual way. This method is very valuable, since complete
accuracy and great speed are obtained at very small cost.
299
APPENDIX 3
GENERAL SUMMARY OF THE PRINCIPAL
NEW FACTS RECORDED
( 1 ) Chromosome counts have been given of all known British members of the Pterido-
phyta and of some non-British species and hybrids together with almost all known
examples of ferns with apogamous life histories.
(2) Allopolyploidy has been demonstrated or strongly suggested in the following
normal taxonomic species :
Dryopteris Filix-mas (L.) Schott sens. strict. emend. (Chapter 4).
Cystopteris fragilis (L.) Bernh. in part (Chapter 7).
Polypodium vulgare L. in part (Chapter 8).
Polystichum aculeatum (L.) Roth (Chapter 9).
Scolopendrium hybridum Milde (Chapter 9).
Woodsia alpina (Bolton) Gray (Chapter 9).
Doodia caudata (Cav.) R.Br. (Chapter 12).
(3) Allopolyploidy has been demonstrated or suspected in all sufficiently investigated
apogamous species (Chapter 11), namely:
Cyrtomium falcatum auct. ( ^C.falcatum Presl, Fortunei }.?im. and caryotideum Presl)
Dryopteris atrata [V^ 3.\\ich) Ching
D. Borreri Newman
D. remota A.Br.
Pteris ere tic a L.
Pellaea atropurpurea (L.) Link
Asplenium monanthes L.
(4) Further work with a view to taxonomic revision is required to establish the nature
of the following new forms :
Diploid Dryopteris dilatata (Chapter 5).
Diploid D. Villarsii (Chapter 5).
Diploid Asplenium Trichomanes (Chapter 6).
Asplenium Adiantum-nigrum var. acutum (Chapter 6).
Various segregants from Polypodium vulgare (Chapter 8).
Various segregants from Cystopteris fragilis (Chapter 7).
(5) The following naturally occurring sterile hybrids have been examined and their
nature discussed :
Dryopteris uliginosa (Newman) Druce (Chapter 5).
D. dilatata (Hoffm.) A. Gray x D. spinulosa (Mull.) Watt (Chapter 5).
'Z). remota Moore', from Windermere (Chapter 5).
300
GENERAL SUMMARY OF THE PRINCIPAL NEW FACTS RECORDED
Asplenium germanicum auct.non Weiss (Chapter 6).
Polystichum illyricum Hahne (Chapter 9).
Woodsia ilvensis (L.) R.Br, x W. alpina (Bokon) Gray (Chapter 9).
Equisetum trachyodpn A.VtY. (Chapter 13).
E. Moorei Newman (Chapter 13).
E. litorale Kuhlw. (Chapter 13).
(6) The following hybrids have been synthesized and their cytology and structure
described:
Dryopteris Filix-mas (L.) Schott s.str. x D. abbreviata (Lam. & DC.) Newm.
(Chapter 4).
Polystichum aculeatum (L.) Roth x P. angulare Presl (Chapter 9).
(7) Lycopodium Selago L. is shown to be problematical and to require further work to
determine its chromosome number and to reconcile the evidence of extensive failure of
pairing at meiosis, suggestive of a hybrid origin, with the known fact of the occasional
occurrence of gametophytes. (Chapter 15.)
(8) The highest chromosome number yet recorded in the plant kingdom has been en-
countered in Ophioglossum vulgatum L., where n = 250-260. (Chapter 16.)
(9) A haploid sporophyte has been obtained by induced apogamy in Scolopendrium
vulgare Sm. (Chapter 12). Other suspected cases of haploids have been shown to admit
of alternative explanations. (Chapters 1 1 and 12.)
(10) A photographic demonstration of the cytology of sporangial development in
apogamous ferns has been given. (Chapters 10 and 11.)
(11) Multivalent chromosome pairing has been demonstrated for the first time in the
Pteridophyta in the autopolyploid series of Osmunda which is described. (Chapter 3.)
(12) Spiral structure of chromosomes has been demonstrated in Equisetum, Psilotum
(rather imperfectly), Hymenophyllum, Todea and Leptopteris, in addition to Osmunda in
which it was already known.
(13) Some new or little-known biological observations are quoted on the cultural
needs of Ophioglossum lusitanicum L. and Isoetes hystrix Durieu (Chapters 15 and 16); on
the prothalHal structure oUsoetes hystrix, Ophioglossum vulgatum and Psilotum (Chapters 14,
15 and 16), and on the coning habits of the European species o^ Equisetum (Chapter 13).
(14) Statistical comparisons are made between the frequencies of polyploidy in the
fern floras of Britain and Madeira and of both with the Flowering Plant floras of
N.W. Europe. From this, conclusions are drawn as to the cause and meaning of
polyploidy (Chapter 17).
(15) Other evolutionary conclusions are discussed in Chapter 17.
301
APPENDIX 4
CHROMOSOME NUMBERS
Complete list of chromosome numbers in the Pteridophyta (new, emended and verified)
introduced throughout the book. The entries under country of origin refer in each case
to the source of the cytological material actually used, though greater detail will in
many cases be found in the partial lists at the ends of the appropriate chapters and
in the text. The treatment of the specific names differs slightly from that of the text
in that preference is in this case given to the most technically correct name known to
me, although where a familiar name was retained in the body of the book but will
now be displaced the synonym is given in brackets to avoid confusion. The use of
inverted commas denotes either an invalid name according to the principles of nomen-
clature, which has been retained from lack of a suitable synonym, or the application
of a valid name to more than one thing, one of which if not both will need a new
specific epithet when further work has been done. The order of arrangement of the
genera of ferns follows Christensen's Index Filicum, supplement 1934, except where
a genus has had to be split to conform to the cytological evidence.
FILICALES
Name
Country of origin
Hymenophyllaceae
Trichomanes:
T. radicans Sw.
Ireland
Hymenophyllum:
H. unilaterale Bory
H. tunbridgense (L.) Sm.
England
Scotland
POLYPODIACEAE
Woodsia:
W. ilvensis (L.) R.Br.
Wales
W. alpina (Bolton) S. F. Gray
Scotland
Cystopteris:
C. montana (Lam.) Desv.
Switzerland
C. Dickieana Sim
Hort. (Scodand)
'C. Baenitzii Dorfl.'
Norway
Greenland
C.fragilis (L.) Bernh.
Britain
Switzerland
Finland
Sweden
Norway
Iceland
Canada
Britain
Switzerland
' C. alpina Desv.'
Switzerland
2n
72
Repro-
duction Chapter(s)
Normal
84
Normal
126
126
17
18
13
Normal
Normal
17
17
c. 41
Normal
7 and 9
(i.e. 41-
c. 82
-42)
Normal
7 and Q
(i.e. 82-
-84)
84
84
84
Normal
Normal
Normal
7
7
7
Normal 7
Normal 7
302
CHROMOSOME NUMBERS
Name
Dryopteris:
D. abbreviata (Lam. et DC)
Newm.
D. aemula (Ait.) O. Kuntze
D. Borreri Newm.
D. remota (A.Br.) Hayek
' D. remota var. Bojdii'
D. remota var. subalpina Borbas
'D. remota Moore'
D. atrata (Wall.) Ching
D. Filix-mas (L.) Schott s.str.
D. spinulosa (Mull.) Watt
D. cristata (L.) A. Gray
D. dilatata (Hoffm.) A. Gray
D. dilatata (form of)
D. Villarsii Woynar
( = Z). rigida (Hoffm.) Underw.
D. uliginosa (Newm.) Druce
D. spinulosa x D. dilatata
D. Filix-mas X D . Borreri
D. Filix-mas x D. abbreviata
Thelypteris:
T. palustris Schott
T. Oreopteris (Ehrh.) C.Chr.
Gymnocarpium:
G. Dryopteris (L.) Newm.
G. Robertianum (Hoffm.) Newm.
Plugopteris:
P. polypodioides Fee
Polystichum:
P. Lonchitis (L.
Roth
P. setiferum (Forsk.) W'oynar
( = P. angulare (Kitaib.) Presl)
P. aculeatum (L.) Roth
P . falcinellum (Sw.) Pr.
P. illyricum Hahne
( = P. Lonchitis x P. aculeatum)
P. aculeatum x P. angulare
Cyrtomium:
C.falcatum (L.f.) Presl
C. Fortunei j.Sm.
C. caryotideum (Wall.) Presl
Athyrium:
A. Filix-femina (L.) Roth
A. alpestre (Hoppe) Rylands
A. flexile (Newm.) Syme
Phyllitis ( = Scolopendrium) :
P. Scolopendrium (L.) Newm.
{ = Scolopendrium vulgare Sm.)
P. hemionitis (Lag.) O. Kuntze
P. hybridum (Milde) Christensen
Country of origin
British Isles
British Isles
British Isles
Switzerland
British Isles
Switzerland
Norway
Ireland
Scotland
Hort. (Switzerland)
England
Botanic Gardens
Britain
Switzerland
Sweden
Britain
Sweden
England
Switzerland
British Isles
Switzerland
Norway
Sweden
England
)
Switzerland
S\\'itzerland
England
England
Ireland
Synthesized
England
England
Britain
Sweden
England
British Isles
Sweden
British Isles
Switzerland
Britain
Switzerland
Britain
Switzerland
Madeira
Switzerland
Synthesized
Hort.
China
Uganda
Britain
Sweden
Scotland
Scotland
Britain
France
Lussinpiccolo
2W
82
82
82
123
123
c. 123
c. 123
c. 164
123
164
164
164
164
82
164
82
164
164
164
205
123
70
68
90
82
82
164
123
123
123
123
123
80
80
72
72
c. 144
41
41
82
Repro-
duction Chapter(s)
Normal 4
Normal 5
Apogamous 5 and 1 1
123 Apogamous 5 and 11
123 Apogamous 5 and 11
c. 123 .\pogamous 5 and 11
c. 123 .\pogamous 5
Irregular Sterile 5
123 .\pogamous 11
82 Normal 5
82
Normal
5
82
Normal
5
82
41
Normal
Normal
5
5
82
Normal
41 Normal 5
Irregular Sterile 5
Irregular Sterile 5
164 Apogamous 5 and 1 1
205 Apogamous 5 and 1 1
Irregular Sterile 4
35
34
80
Normal
Normal
Normal
5
5
c. 80 Normal
(i.e. 80-84)
90 Apogamous
41
41
82
164
Irregular
Normal
Normal
Normal
Normal
Sterile
6 and 9
6 and 9
6 and 9
17
9
Irregular Sterile
123
123
123
40
40
36
36
c. 72
Apogamous 10 and 1 1
Apogamous i o and 1 1
Apogamous 10 and 11
Normal
Unknown
Normal
6
6
Normal 7, 9 and 1 2
Normal
Normal
9
9
303
CHROMOSOME NUMBERS
Name
Asplenium:
A. fontanum (L.) Bernh.
A. viride Huds.
A. marinum L.
A. Adiantum-nigrum L. var. acutum
(Bory) PoUini
A. Petrarchae DC.
A. ruta-muraria L.
A. septentrionale (L.) Hoffm.
A. lanceolatum Huds.
A. Adiantum-nigrum L.
'A. Trichomanes L.'
A. aethiopicum (Burm.) Bech.
A. germanicum auct.non Weiss
{ = A. Breynii Retz.)
A. monanthes L.
Ceterach:
C. officinarum Lam. & DC.
C. aureum (Cav.) v. Buch
Blechnum:
B. spicant (L.) With.
Doodia:
D. caudata (Cav.) R.Br.
Pellaea:
P. atropurpurea (L.) Link
Cryptogramma:
C. crispa (L.) R.Br.
Adiantum:
A. capillus-veneris L.
A. reniforme L.
Pteris:
P. cretica L.
var. albolvieata Hook.
P. cretica L.
Pteridium:
P. aquilinum (L.) Kuhn
Polypodium:
P. vulgare L. var. serratum (Willd.
Milde'
P. vulga
'P. vulgare L.'
P. virginianum L.
'P. vulgare L. var. occidentale Hook.'
Country of origin
Switzerland
Britain
Britain
Madeira
France
Britain
Britain
Switzerland
Finland
England
England
England
Switzerland
France
Wales
Madeira
Wales
Italy
Switzerland
Sweden
Madeira
England
France
Teneriffe
Britain
Botanic Garden
Induced
Induced
California
England
Ireland
Italy
Spain
Madeira and TeneriflTe
Italy
Hort.
Uganda
Britain
Malay
France
Switzerland
Italy
England and Ireland
British Isles
Scandinavia
Switzerland
France
British Isles
Western Europe
Nova Scotia
Western N. America
2«
n
Repro-
duction
Chapter(s)
72
72
72
72
36
36
36
Normal
Normal
Normal
Normal
6
6
6
6
144
72
72
72
Normal
Normal
Normal
6
6
6
144
72
72
72
Normal
Normal
Normal
6
6
6
108
36
144
Irregular
Normal
Normal
Sterile
6
17
6
108
108
Apogamous
II
72
Normal
6
72
Normal
6
68
34
Normal
7
c. 70
Over 200
65-70
Irregular
Unknown
Normal
Sterile
Unknown
12
12
12
87
87
Apogamous
II
—
60
Normal
7
60
30
Normal
7
58
c. 90
c. 120
74
150
58
c. 90
c. 120
52
37
74
III
37
37
Normal
17
Apogamous 10 and 11
Apogamous 1 1
Apogamous 1 1
Normal 7
Normal
Normal
Normal
Normal
Normal
8
8
OSMUNDACEAE
Todea:
T. barbara (L.) Moore
Leptopteris:
L. Frazeri (Hk. & Grev.) Presl
L. hymenophylloides (A. Rich.) Presl
Botanic Gardens
Botanic Gardens
Botanic Gardens
22
22
22
Normal
Normal
Normal
16
16
16
304
CHROMOSOME NUMBERS
Name
Repro-
Leptopteris (cont.)
Country of origin
2n
n
duction
Chapter(s)
L. superba (Col.) Presl
Botanic Gardens
—
22
Normal
16
Osmtinda:
0. regalis L.
British Isles
44
22
Normal
3
Synthesized
66
Irregular
Sterile
3
Synthesized
88
44
Normal
3
0. cinnamotnea L.
Botanic Gardens
—
22
Normal
16
0. Claytoniana L.
Botanic Gardens
—
22
Normal
16
0. gracilis Hort.
Botanic Gardens
—
22
Normal
16
0. palustris Hort.
Botanic Gardens
—
22
Normal
16
O.javanica Bl.
Malay
—
22
Normal
16
Ophioglossaceae
Ophioglossum:
0. vulgatum L.
England
—
250-60
Normal
16
0. lusitanicum L.
Guernsey
—
125-30
Normal
16
Botrychium:
B. lunaria (L.) Sw.
England
EQ.UISETALES
45
Normal
16
Equisetum subgenus Eu-equisetum:
E. arvense L.
England
—
Prob. 108
Normal
13
E. sylvaticum L.
England
—
Prob. 108
Normal
13
E. maximum Lam.
England
—
Prob. 108
Normal
13
E. pratense Ehrh.
Hort.
—
Prob. 108
Normal
13
E. palustre L.
England
—
Prob. 108
Normal
13
E. limosum L.
England
—
Prob. 108
Normal
13
E. litorale Kuhlvv.
Ireland
—
Irregular
Sterile
13
Equisetum subgenus Hippochaete:
E. ramosissimum Desf.
Italy
—
Prob. 108
Normal
13
E. hiemale L.
England
—
Prob. 108
Normal
13
E. robustum A. Br.
Botanic Gardens
—
Prob. 108
Normal
13
E. scirpoides Michx.
Norway
—
Prob. 108
Normal
13
E. variegatum Schleich.
British Isles
—
Prob. 108
Normal
•3
E. trachyodon A.Br.
Ireland
Prob. 2i6
Irregular
Sterile
13
E. Aloorei Newm.
Ireland
PSILOTALES
Irregular
Sterile
13
Psilotum:
P.flaccidum Wall.
Botanic Gardens
—
52-54
UnknowTi
14
P. nudum (L.) Beauv.
Ceylon
C. lOO
Unknown
14
Malay
C. 200
c. 100
Normal
14
Australia
New Zealand
Tmesipteris:
T. tannensis (Spreng.) Bernh.
Lycopodium:
L. imindatum L.
L. clavatum L.
L. amiotinum L.
L. alpinum L.
L. Selago L.
Isoetes:
I. lacustris L.
/. echinospora Durieu
/. hystrix Durieu
Selaginella:
S. spvmlosa A.Br.
S. helvetica (L.) Link
S. denticulata (L.) Link
Botanic Gardens Over 400
LYCOPODIALES
Over 200 Unknown
14
Scotland
— .
78
Normal
15
England
68
34
Normal
15
England
c. 68
c. 34
Normal
15
Switzerland
Sweden
Britain
f. 48
24-25
Normal
15
Britain
At east
Irregular
Proble-
15
Sweden
260
matical
Britain
Not less
than 100
54-56
Normal
15
Ireland
Not less
than 100
—
Normal
15
Britain
20
10
Normal
15
Morocco
England
18
9
Normal
15
Switzerland
18
—
Unknown
15
Italy
18
9
Normal
15
MPC
305
20
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310
INDEX
Abnormal life histories, 20-2; chs. 10 and 11.
See also Dryopteris Borreri, D. remota, and D.
Phegopteris
Adiantwn capillus-veneris, 124; and Y'lgs. 123/), 125
A. reniforme, 283
Allen, 159-60
Allopolyploidy
definition, 5
Doodia caudata, 202
frequency, 286
Male Fern, 54
Polystichum aculeatum, 153
summary, 282-3, 286
watercress {Nasturtium), g
Woodsia alpina, 151
Alternation of generations, 18-19; ^"^ Figs- 5? 6
Amitosis, 166; and Fig. 169
Anaphase, definition, 34
Andersson-Kotto and Gairdner, 124, 182
Anemorrhoea, 276
Aneuploidy, 5, 14-16, 125, 286, 287
Apogamy
characteristics of, 57-8; and Figs. 42, 43
cytology of, chs. 10 and 1 1
definition, 20-1; and Fig. 7
in Asplenium monanthes, 183, 195
in Cyrtomium, 159-70, 177-82
in Dryopteris atrata, 158, 160, 167
in D. Borreri, 54-61, 185-94
in D. remota, 71-5, 79, 185
in Pellaea, 158, 184-6
in Phegopteris, 82-3, 183-4
in Pteris cretica, 167, 17 1-7
origin of, 194-5, 285, 291
See also Induced apogamy
Apospory, 21-2; and Fig. 8
in Doodia, 203
in Osmunda, 26-8; ani Figs. 11, 12
'peculiar' Scolopendrium, 124
Archegonium, 17
in Equisetum, 19; and Fig. 6c
in Ophioglossum, 266-8; and Figs. 265a, 266
Ashby, xiii, 28, 198, 200, 203, 255, 259
Aspidiurn aculeatum, 197. See also Polystichum
aculeatum
A. angulare, 197. See also Polystichum angulare
A. falcatum, 158-9. See also Cyrtomium
A. frondosum, 197
Asplenium, 98-106, 108, 122, 124, 281, 287
A. Adiantum-nigrum, 99-100; and Fig. 92
var. acutum, 100
A. aethiopicum, 283
A. alternifolium, 100
A. Breynii, 100
'Asplenium Filix-foemina' var. clarissima ]ones, 21-2;
and Fig. 8b
A.fontanum, 100, 102; andY'ig. 96
A. furcatum, 283
A. germanicum, 100-6, 285; and Figs. 97-9
A. lanceolatum, 98-101 ; and Fig. 956
A. marinum, 98-101; and Figs. 89, 91, 93c
A. monanthes, 158, 183, 195
A. Murbeckii, 103
A. Petrarchae, 100, 102; a«^ Fig. 96
A. ruta-muraria, 99-103; and Fig. 95 a
A. septentrionale, 98, 100-3; ^^^ Figs. 93, 94,
97
A. Trichomanes, 99-109; a«^ Figs. 93, 97, 101-3
A. viride, 98-101; and Figs. 89, 90, 93, 94
Afhyrium, 93-8, 108, 291
A. alpestre, 94-5; and Figs. 82-4
A. Filix-femina, 93-4; and Figs. 81, 82
var. clarissima, 21; and Fig. 86
var. percristatum, etc., 197
A. flexile, 95-9; and Figs. 82c, 85-8
A. niponicum, 197
Australia, 238-40
Autopolyploidy
in Biscutella, 8
definition, 5
evolutionary significance, 42, 285, 287
in Osmunda, ch. 3; 285
in Psilotum, 240, 285
Babcock, 5, 14-15
Baragwanathia, 16, 245
Barber, 233, 238-9, 240, 243
Bateson, i, 14
Bathypteris, 276
Beech Fern, 63, 81-3. See also Phegopteris
Belling, 2
'Beringia', 140
Bisby, 48
Biscutella, 5, 6-8, 14-15; and Figs, i, 2
Blackburn and Harrison, 5
Bleaching (photographic), 298
Blechnum, 122-4
B. spicant, Fig. 1 23/
Botrychium, 269-70; and Figs. 267-9
Bower, xii, 22, 24, 63, 93, no, 124, 128, 262
Brassica, 13
British flora, 88, 281-2, 286
Bruchmann, 245, 265-6
Bryophyta, 42
3"
INDEX
Calamites, i6, 209
Canada, 121, 138; and¥i%. 119 c
Cardamine, 9, 1 1
Ceterach, 106-8, 142-7
C. aureum, 108
C. qfficinarum, Figs. 104-6, 147
Chiasma, 35
China, 178, 180; andY'ig. 183A
Christensen, 24, 63, 80-1, no, 128
Chromosome morphology, 5. See also under shape
Chromosome numbers
evolutionary significance, 4, 171, 287-9
lists, 87, 109, 126, 195, 231-2, 261. 280,
Appendix 4, pp. 302-5
literature, ix, 13, 215, 281-2
Chromosome pairing
illustrative examples, 7, 9
meaning of, 5
multivalent pairing, 34, 36, 175-6, 240, 285
stages of, 34
use of in analysis, 5-6, 171
Chromosome shape, 34-5, 215, 246, 255, 286
Chromosome size, 273, 287
Chromosome structure. See Spiral structure
Clapham, xii, 63
Clausen, Keck and Hiesey, 71, 290
Coenopterideae, 16
Coenospecies, 44, 71
Copeland, xii, 24, 81, no, 262
Crambe, 15
Creighton and McClintock, 2
Crepis, 5, 14-15
Cruciferae, 13-16, 25, 285-7
Cryptogramma crispa, 125; and Fig. 123c
Cylindrical processes in induced apogamy, 196-7
Cyrtomium, 22, 80, 158-70, 177-82
C. caryotideum, 162, 177, 179; aW Figs. 179, 183c,
184
C.falcatum, Figs. 163-8, 18 1-3
var. Rockfordii, 178-80; aW Figs. 181, 182
C. Fortunei, 162, 169, 178; and Figs. 170, 171, 180,
183b, 185
Cystopteris, 112-22, 263, 281-2, 285, 287
C. alpina, 1 12-16; ana' Figs, wib, 112, 113
C. Baenitzii, 1 17-21 ; and Fig. 1 17c
C. Dickieana, 112, 1 17-18; aW Figs. 1 14-18
C.fragilis, 1 12-22; aW Figs, no, ni, n8-2i
C. montana, \\2, 122, 282; an^ Figs. i2od, 122
C. regia. See C. alpina
Cytogenetics, history, 2
methods, 4-6, 284
Cytological technique, xi, n2, 284, 293-7
Darwinism, 2-3, 290
Diakinesis, 34, 36
Diplotene, 34
Doodia caudata, 197-204, 288; and Figs. 202-7
Dopp, 59, 72, 159-60, 166, 171, 193
Drepanophycus, 16, 245
Drosera, 5
Drosophila, 2, 5, 14
Druce, 63, n7
Druery, 48
Dryopteris, ch. 5; 281, 285, 287, 291
relationship to Athyrium, 93-4, 108
subdivision of genus, 81, 85
D. abbreviata, 46-53; and Figs. 29, 31, 32, 34.:,
36, 40, 42^
D. aemula, 63-5; and Figs. 50, 51, 53a, 71 ^
D. atrata, 158, 160, 167, 180-3; ««^ FigS' i6ga.
I 86-90
D. Borbasii, 73
D. Borreri, 54-61, 158-9, 161, 167, 171, 186-94,
285-6; and Figs. 7, 29, 41-9, 1690'-^, 195-201
var. polydadyla, 58, 158-60, 167, 186-7; «"^
Figs. 7a, 44, 45, 47, 1690'-^, 195, 196
D. Boydii, 73-4; and Fig. 60
D. cristata, 65-71; and Figs. 53^, 54^/, 56
D. cristata x D. spinulosa. See D. uliginosa
D. dilatata, 44, 65-72, 75-8, 86, 286; and Figs. 53^,/,
54^ 71C
diploid form, 75-9, 86, 282; and Figs. 53^, 62-4
D. dilatata x D. spinulosa, 69-71 ; and Figs. 54c, 57^
D. Filix-mas, 44-6, 49, 52-4, 75, 286; and Figs.
29, 30, 32, 34c, 35, 39, 42c-^, 43
D. Oreopteris, 79-81; and Figs. 65, 66, 71a
D. Phegopteris, 81-3, 183-4; and Figs. 69-71
D. polypodioides. See Gymnocarpium
D. remota, 71-9, 187; and Figs. 58-61, 1696, 194
var. subalpina, 73-4; and Fig. 60
D. rigida, 75. See also D. Villarsii
D. spinulosa, 65-72, 75, 286; and Figs. 53c, 54a, 55
D. Thelypteris, 79-81, 85; and Figs. 67, 68
D. uliginosa, 68-71, 285; and Figs. 54b, 57a
D. Villarsii, 65-7, 86, 286; and Figs. 52, 536 and
Frontispiece
Duncan, 197-203
Dysploidy, 5
Ecospecies, 71, 140
Ecotype, 14, 71
Equisetales, 17, 22
Equisetites Hemingwayi, 209
Equisetum, 16-19, 22; ch. 13; 241, 285-6, 288-9
alternation of generations, 17-20; and Figs. 5, 6
chromosome number, discussion, 222-3, 285,
289; list, 231-2
chromosome shape, 215
chromosome sizes, 215-18
chromosome structure (spiral), 215; andVig. 228
chromosome swelling, 220-1
cone buds, 213-14; and Figs. 5, 212
cultivation, 212
fertilization, 19, 268; andYig. 6c
312
INDEX
Equisetiim {cont.)
gametophytes, 19, 223; andY'ig. 6
geographical distribution, 210, 227
geological history, 23, 209
hybrids, 224-31 ; and Figs. 224-8
previous chromosome counts, 215
seasonal periodicity, 212-14
E. arvense, 210, 212, 214-15, 219-21, 225, 231;
and Figs. 217-19
E. hiemale, 210, 212, 218, 230, 232; aW Figs. 215 a,
216
E. limosum, 18, 210, 212, 215, 224-5, 228, 231;
and Fig. 222 a and c
E. litorale, 212, 224-9, 231, 285; and Figs.
224-7
E. maximum, 2 10, 2 1 2-15, 23 1 ; aWFigs. 2 12, 220, 22 1
E. Moorei, 229-32, 285; and Figs. 225, 2276
E. palustre, 18, 210, 212, 215, 232; an^ Fig. 222^
E. pratense, 210, 212, 214-15, 231
E. ramosissimum, 210, 213, 218, 230, 232
E. robustum, 18, 211-12, 221, 232; and Figs. 213,
2140. 217 i
E. scirpoides, 210, 212, 218; an</ Figs. 211, 215^
E. sylvaticum, 210, 212, 214-18; and Fig. 6
E. trachyodon, 211-12. 218, 227-32, 285; and
Figs. 225, 227, 228
E. variegatum, 210, 212, 218, 230-1; aW Figs. 213^,
214A
Erophila, 13
Eusporangiatae, 262
Farmer and Digby, 159, 187
Ferns. See Filicales
Fertilization, 17, 19
in Equisetum, 19, 268; and Fig. 6
in Ophioglossum, 268
Feulgen technique, xi, 221, 293, 295, 297
Filicales, 24, 284, 289
Finland, 120-1
Fixation, 26, 152, 174, 215, 250; andY'ig. 173
Flowering Plants
evolutionary mechanisms, 13-16, 290
examples, ch. i
frequency of polyploidy, 281-2
life history, 16, 18
summary of conclusions, 283-7
France, 7, 47, 100, 105, 108, 132, 145
Gametophyte, definition, 18. See also under
Prothallus
Gerassimowa, 14
Giant mother cells, 166, 168
Glaciation, 140-1, 283-4, 290. See also Ice Age
Goldschmidt, 4
Greenland, 118, 121, 140; and Fig. wjb
Gregoire, 2
Guernsey, 255-7, 262-3; and Figs. 254/!', 261a
Gymnocarpium, 85-7, 94
G. Dryopteris (Oak Fern), Figs. 71-3
G. Robertianum, Fig. 726
Haploid, definition, 5
Scolopendrium, 205-6; and Figs. 208-10
Heilbronn, 103
Heim, 197
Helminthoslachys, 262
Heterospory, 254-5, 291
Heterotype, 34
History of biology, 1-3
Holloway, 233, 235-6
Holly Fern. See Polystichum Lonchitis
Holttum, xii, 81, 124
Homotype, 34
Horsetails. See Equisetum
Hulten, 140, 147
Huxley, 65
Hybridization, methods, 48
importance in evolution, 283
summary, 285
Hybrids, sterility of, 51
between apogamous and sexual species, 171
in apogamous ferns, 194
synthesized hybrids, 49-53, 154-6, 191
wild hybrids, Asplenium, 100-6; Dryopteris,
59-61, 70-5, 1 9 1-3; Equisetum, 224-31; Lyco-
podium, 251-2; Woodsia, 147-51
Hydropterideae, 17
Hymenophyllaceae, 24, 262, 270-3, 280
Hymenophyllum, 262, 270, 272-4, 280, 286-7
H. tunbridgense, 272-4; and Figs. 273-5
spiral structure, 273-4; '^"^ ^ig. 275
H. unilaterale, 272-4; and Fig. 274a
Ice Age, 9, 283-4, 290. See also Glaciation
Iceland, 121
Induced apogamy, 6, 21, and ch. 12
in Doodia caudata, 198-204; and Fig. 202a
in Scolopendrium vulgare, 204-7
Indusium, shape, 46, 54
absence of, 63, 81, 94-5, 128
in Asplenium, 93
International Rules of Nomenclature, 48
Ireland, 48, 59, 69, 73, 134, 212, 223-31, 254, 270;
and Figs. 224-8, 270
Isoetes, 17, 22, 244, 253-9, 286-9
/. echinospora, 254-5
/. hystrix, 255-9; «"^ Figs. 254-7
/. lacustris, 253-5; and Figs. 252, 253, 256a
Italy, 104, 125, 132, 174, 212-13, 259
Janssens, 2
Jordanian species, 1 3
Karpechcnko, 13
313
INDEX
Kestner, 103, 144
Kidston, 209
Kidston and Gwynne-Vaughan, 276
Lamarck and de Candolle, 47-8
Lamarckism, 3, 290
Lang, xiii, 21, 26, 124, 196-7, 204-5, 245-6
Lastrea propinqua. See Drjopteris abbreviata
L. pseudo-mas. See Dryopteris Borreri
var. cristata-apospora Cropper, 22
L. remota, 72-3. See also Dryopteris remota
Leptopteris, 274, 276, 278-80, 286
L. Frazeri, 278-9; and Fig. 2786
L. hymenophylloides , 278-9; and Fig. 278c
L. superba, 278-9; and Figs. 278^/, 279c
Leptotene, 34; andY'ig. 19
Limestone Polypody, 63, 81, 83-5; an^Fig. 12b
Love and Love, ix, 281-3, Table 8
Luerssen, 73, 143
Lussino, 142-5
Lycopodiales (Lycopods), 16, 17, 22; ch. 15; 285
Lycopodium, 22-3, 244-54, 286
prothalli, 245-6
L. alpinum, 245, 248-9; and Figs. 245, 246
L. annotinum, 244-8, 252; an^ Figs. 247, 248, 251
L. clavatum, 244-9; <^^^d Figs. 241, 242, 244, 246 c/, e
L. inundatum, 245-7; ^"^ Figs- 243, 246^
L. Selago, 245-6, 250-2, 285; and Figs. 249, 250
Macroevolution, 4, 12, 16, 289-91
Madeira, 192, 195; fern flora of, 282-3, Table 9
Maiden Hair Fern. See Adiantum
Malay, 125, 239, 276; aW Fig. 234/)
Male Fern. See Dryopteris Filix-mas
Marattiaceae, 262
Marchals, 42
Marsh fern, 79, 81. See also Dryopteris Thelypteris
and Thelypteris
Marsiliaceae, 24
Martens, Fig. 142
Megaphyllous, definition, 209
Meiosis, history and definition, 2
illustrations of stages, 32, 34, 36, 39
place in life history, 18
stages, 32-4
Mendel, i
Mendelism, in Scolopendrium, 124
in apogamous ferns, 193-4
Metaphase, definition, 33
Microevolution, 4, 290
Microphyllous, definition, 209
Milde, 143-5, 212, 224
Mitosis, 2
Morocco, 257-8; and Fig. 255
Multivalents
in Biscutella, 7-8
in Osmunda, 34-9
in Psilotum, 239
in Pteris cretica, 175-6
in Watercress, 9-1 1
loss of, 285
Mutation, 3-4, 56, 180, 194, 286, 288-91. See also
Saltation
Nasturtium, 6, 9-12; awif Figs. 3, 4
Nephrodium dilatatum, 197. See also Dryopteris
dilatata
Nephrodium hirtipes, 158-9. See also Dryopteris atrata
Nephrodium molle, 197
N. Oreopteris, 197. See also under Dryopteris
Nephrodium pseudo-mas var. polydactyla, 159. See
also Dryopteris Borreri var. polydactyla
Newman, 47, 55, 1 10, 1 16-17
New Zealand, 233-9
Nordhagen, 188; and Figs. 49, 197
Norway, 75, 118, 120-1, 128, 147, 189, 232;
and Figs. 63, 1 17c
Oak Fern, 63, 81-5. See also Gymnocarpium
Okabe, 233, 239
Ophioglossaceae, 17, 24, 262, 280
Ophioglossum, 262-9, 273, 280, 285, 287-8; and
Figs. 261-6
0. lusitanicum, 263-4, 268-9; and Figs. 261-3
0. vulgatum, Fig. 261^
archegonium, 266-8; and Fig. 265a
chromosome number, 264, 268-9, 285
fertilization, 267-8
prothalli of, 264-8; and Figs. 264-6
spermatozoid, 266; and Fig. 265A
Orthogenesis, 291
Ortus Sanitatis, 127; and Yig. 126
Osmunda, ch. 3; 274-80, 285-9; and Figs. 7, 8,
1 1-28, 276-9
apogamy in, 20; and Fig. 76, c
apospory in, 21, 26-g; «««/ Figs. 8, 11, 12
polyploidy in, 26-43, 285; andYigs. 13-28
spiral structure of chromosomes, 34-5, 279;
and Figs. 21, 279
O. cinnamomea, 276; and Fig. 277a
0. Claytoniana, 276; and Fig. 2']']c
O. gracilis, 277-9; '^"^ Figs- 2776, 2796
O. interrupta. See 0. Claytoniana
0. javanica, 277-9; arid Fig. 277^
0. palustris, 275, 277-8; and Fig. 2Tjd
0. regalis, ch. 3; 276
Osmundaceae, 24, 262, 274-80
fossil, 276
primitive nature of, 279, 288
Osmundites, 276
Owen (Richard), 3
Pachytene, 34; and Figs. 20, 185a
Parallel evolution, 81, 108, 195, 290-1
314
INDEX
Parthenogenesis, 6, 20
Payne, 65
Pellaea atropurpurea, 158, 184-6; and Figs. 191-3
Phegopteris, 82-3, 87, 158, 161, 183-4, 286.
See also under Dryopteris Phegopteris
Photography, xi, xii, 298-g
Phyllitis, 122. See also Scolopendrium
Phylloglossum, 22
Phylogeny, Pteridophyta, 23 ; and Fig. g
ferns, 24, 89; and Figs. 10, 74
Horsetails, 209
Osinundaceae, 276
Polyploidy
chromosome pairing in, 7-g, 35-8
definition, 5
frequency in Cruciferae, 13
in apogamous ferns, ch. 11
in British ferns, 282, Table 9
in European Floras, 281-2, Table 8
in Madeira ferns, 282-3, Table 9
in Osmunda, ch. 3
in Psilotum, 238-40
production of, 22, 26, 28, 282-4
summary, 285-7, 291
Polypodiaceae, 24, 128
Polypodium, chap. 8; 281, 283, 285-7, 291
P. virginianum, 138-40; and Figs. 141, 142
P. vulgare, ch. 8; and Figs. 126-44
hexaploid, 134; aW Figs. 127-31, 134, 135^
pentaploid, i '^6; and Figs. I'^^d, 137, 138
tetraploid, 132; aW Figs. 130-2, 135c, 136
triploid, 136; andFigs. 135A, 139
var. occidentale, 138-9, 141; and/ Figs. 140, 143-4
var. semilacerum, Fig. 123a
var. serratum, 132-4; aW Figs. 130, 131, 133, 135
Polystichum, 80, 88-93, 15 1-7
P. aculeatum, 88-92, 151-7, 286; and Figs. 77,
796, 80, 156, 157
P. aculeatum X P . angulare, 154-7; andFigs. 160-2
P. angulare {P. setiferum), 89-92, 154-6; and Figs. 76,
79«
P.falcinellum, 283
P. illyricum { = P. LonchitisxP. aculeatum), 150-4,
285; andFigs. 155-9
P. Lonchitis, 88-92, 151-4; and Figs. 75, 78, 79c,
156, 157
P. setiferum, 89. See also under P. angulare
Praeger, 61, 72-3, 224-5, 228
Prophase, definition and stages, 33-4
Prothallus
apogamous prothalli, 20, 22, 57-8, 83, 158, 196;
and Figs. 7, 42, 43
definition of, 17-19
in Equisetum, 19; and Fig. 6
in Isoetes hystrix, 258-9; and Fig. 257
in Ophioglossum, 264-8; and Figs. 264-6
in Osmunda, 26-9, 40; and Figs. 15-17, 26
in Psilotum, 235-7; ^"^ Figs. 231-3
in the Male Fern complex, 57; and Fig. 42
saprophytic prothalli, 17, 235-7
Pseudofertilization, 159
Psilophytales, 16-17
Psilotales, 16-17, 22-3; ch. 14; 285, 288
Psilotum, 22, 233-40, 285
polyploidy in, 238-40, 285
spiral structure, 240, 286
tripolar spindle, 238
vascular prothalli, 235-7, 243
P.Jlaccidum, 235, 239-41; and Figs. 230, 236, 237
P. nudum, 235-41 ; and Figs. 229, 231-5
P. triquetrum. See P. nudum
Pteridium aquilinum, 124-5; and Fig. i2^d
Pteridophyta, aim of the enquiry, 13, 16, 25
characters of, 16-20
enumeration of, 16-24
phylogeny, 23-4
Pteris cretica, 22, 158, 161, 168, 17 1-7, 285; and
Figs. 169c, 172-8
Pteropsida. See Filicales
Quadrivalents, in Osmunda, 34, 36-8; andFigs. 20f.
'22d, 24
Quillwort. See Isoetes
Ramenta, 46-7, 49, 54, 75
Raphanohrassica, 13
Restitution nucleus, 165-6
Rosa, 5
Rosenberg, 5
Royal Fern. See Osmunda
Rumex, 5
Runmaro, 104, 147
Russia, 14, 120, 140
Saltation, 170, 288
Saprophytic prothalli, 17, 236, 264-8; and Figs.
231-3^ 264-6
Scolopendrium { — Phyllitis), 122-4, i42-7> 204-7
S. hemionitis, 143, 145-6; a W Figs. 147, 149-51
S. hybridum, 142-6, 157; andFigs. 145, 146, 148
S. vulgare, 123-4, H3, '45-7; andFigs. 123^, 124,
148c
induced apogamy in, 204-7; and Figs. 208-10
Scotland, 48, 69, 93, 95-8, no, 112, 118, 246;
andFigs. 83-6, 1076, 119
Segmental interchange, 53
Selaginella, 17, 22-3, 259-61, 273, 287-9; and
Figs. 258-60
Shaw, 10
Silhouettes, technique, 299
Sledge. See Athyriumjlexile and Lycopodium annotinum
Sorus, use of in classification, 108
parallel evolution, 291
Spain, 7, 125
315
INDEX
Spartina, 4, 6
Species concept, 51
Spermatozoid (Frontispiece), 17, 19, 31, 266;
and Figs. 6, 17, 265 b
Spindle, 34, 38, 164; and Fig. 167a
tripolar, 238-40; and Figs. iSge, 234c
Spiral structure of chromosomes, 286
description of in Osmunda, 34-5; and Fig. 21
in Equisetum, 215; and Fig. 228
in Hymenophyllum, 273; and Fig. 275
in Leptopteris, 279; and Fig. 279
in Psilotum, 240-1; and Fig. 236^
in Todea, 279; aW Fig. 279
Spleenwort. See Aspleniiim
Sporangium, 16-18, 32; fl«(/ Figs. 5c, 18
development of, 162
Doodia, apogamous and normal, 201; and Fig. 205
in apogamous ferns, 163-9
on prothalli, 196-7, 205
Spore, 16, 18
germination in polyploids, 39-40; and¥igs. 26, 27
in apogamous ferns, 22, 159, 163-8
in Cystopteris, 1 1 8-2 1 ; and Fig. 1 1 8
in Dryopteris Borreri, Fig. 20 1
in Equisetum, 18; and Fig. 5
sizes in D. Filix-mas complex, 47, 55; and Figs.
39-41
Sporophyte, 18
Squash techniques, 114, 178-9, 221
details of the methods, 295-7
examples of their use, 53, 63, 79, 100, 259
Staining methods, 294-7
Stange, 58, 198
Stansfield, 69, 72-3, 144
Steil, 159-60, 180
Stern, 2
Strasburger, 2
Sweden, 75, 109, 120-1, 126, 147-51, 248, 252; a?id
Figs. 62, 69c, 97^, 119a, 247
Switzerland, 7, 55-6, 59, 65, 69, 75, 92, 100,
114, 121, 126, 133, 151-4, 189-91,247-8,259;
and Figs. 64, in, iigd, 155-7, 258
Symbiosis, 17, 236, 265
Tavel, von, 55-9
Technique, xi, 112, 293-9
Telophase, definition, 34
Thamnopteris, 276
Thelypteris, 81, 85, 87, 108. See also Dryopteris
Thelypteris
Tmesipteris, 22, 233, 235-36, 240-3, 285; and
Figs. 238-40
Tobgy, 14
Todea, 274, 276, 278-80, 286
T. Barbara, 278-9; and Figs. 278a, 279a
Tracheids, 16
in apogamous prothalli, 197
in Psilotum prothalli, 236-7
Trichomanes, 262, 270, 280
T. radicans, 270-2; and Figs. 270-2
Triploids
definition, 5
D. Filix-mas hybrid, 49-53; and Figs. 32-4, 37, 38
in apogamous ferns, 195
in Asplenium germanicum, 1 04-5
in Biscutella, 7, 9
in Dryopteris Borreri, 59-60, 189-92
in Osmunda, 28-30, 34-42
in Polypodium, 136, 138
in watercress (Nasturtium) , 9-1 1
Polystichum hybrids, 152-7
Woodsia hybrid, 147-51
Trivalents, in Biscutella, 7-9; and Fig. 2e
in Cyrtomium, 179-80; and Fig. 185
in D. Filix-mas hybrid, 53; and Figs. 37, 38
in Osmunda, 36-9; and Figs. 20b, 22c, 23, 27c
in Pteris cretica, 175; and Fig. 177
Turesson, 44, 71, 290
Uganda, 174-6, 177-9; a«^ Figs. 178, 179
Univalents
in apogamous ferns, 175, 183, 185, 187, 192;
and Figs. 171, 173c, 176, i-j^b, 189a, 192c,
194^, 196^*, 199-201
in Asplenium germanicum, Figs. 99, 100
in Biscutella, 7; and Fig. 2e
in D. Filix-mas x D. abbreviata, 53 ; and Figs. 37, 38
in hybrid Equisetum, 228-31; and Figs. 226-8
in induced apogamy. Figs. 206, 209, 210
in Lycopodium Selago, 250-1; and Figs. 249, 250
in Ostrmnda, 37-9; and Figs. 22c, 23, 25
in other hybrid ferns. Figs. 57, 59
in triploid Polystichum, Figs. 158, 159, 161, 162
in triploid Woodsia, 149; an^ Figs. 153, 154
in watercress, g; and Figs. 3, 4
Variation, 1-3
Vries, de, 3
Wales, 48, 59, 103-7, iiO' 134. 251, 255; and Figs.
97-9, loib, 1036, 107a, I lie
Watercress, 9-1 1 ; and Figs. 3, 4
Wettstein, von, 42
Winge, 13
WoUaston, 47-8, 55
Woodsia, 80, 1 10-12, 147-51
W. alpina, 110-11,147,1 49-5 1 , 286 ; and Figs. 107-9,
i52e
W. alpina X W. ilvensis, 147-51; and Figs. 152-4
W. ilvensis, iio-ii; a W Figs. 107-9, '52 a
Yamanouchi, 197
Zalesskya, 276
Zea, 2
316
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